Smoke rises following an Israeli air strike on Gaza City, Oct. 9. EPA-Yonhap

Israeli Prime Minister Benjamin Netanyahu declared on Monday that Israel's fierce offensive against Hamas in the Gaza Strip has "only started."

Netanyahu delivered the pronouncement in a nationally televised address as Israel pressed ahead with a third day of heavy airstrikes in Gaza following Hamas' unprecedented and deadly incursion into Israel.

"We have only started striking Hamas," he said. "What we will do to our enemies in the coming days will reverberate with them for generations."

Israel formally declared war on Sunday. The hostilities so far have killed around 900 people in Israel and more than 680 people in Gaza, according to authorities on each side.

At least 11 U.S. citizens have been confirmed dead in the surprise Hamas attacks, U.S. President Joe Biden said Wednesday.

Biden also said the U.S. government believes it is "likely" that Americans are among those currently being held hostage by Hamas militants, while other U.S. citizens are still unaccounted for after the deadly assault.

"My heart goes out to every family impacted by the horrible events of the past few days," Biden said in a statement. "The pain these families have endured, the enormity of their loss, and the agony of those still awaiting information is unfathomable."

He stressed that the State Department is offering assistance for U.S. citizens who are currently in Israel, and air and ground options to leave the country are still available for those who choose to do so. He also said federal law enforcement officials are "closely monitoring" potential domestic threats stemming from the weekend attacks.

British Prime Minister Rishi Sunak joined a large crowd gathered for prayer at a London synagogue on Monday to underline his solidarity with Israel.

Addressing the congregation at Finchley United Synagogue in north London, Sunak condemned Hamas' bloody incursion into Israel on Saturday as "evil."

"There are not two sides to these events. There is no question of balance. I stand with Israel," he said. "The United Kingdom stands with Israel against this terrorism today, tomorrow and always."

He also sought to assure Britain's Jewish community that he will do everything to protect them amid concerns about the rise of antisemitism.

Thousands of pro-Israel and pro-Palestinian protesters held vocal but peaceful gatherings Monday, at a historic French plaza and outside Israeli embassies in London and Athens, Greece.

Several thousand people rallied in Paris in support of Israel, marching to the Trocadero Plaza overlooking the Eiffel Tower. The monument was lit up in the colors of the Israeli flag, with a white Star of David in the middle, in a gesture of solidarity announced by Paris Mayor Anne Hidalgo.

In Athens, an estimated 250 people, mostly Palestinian expatriates and members of Greek left-wing groups, held a peaceful protest outside the Israeli embassy to show solidarity with Palestinians. Protesters waved Palestinian flags and chanted slogans, while a strong police contingent blocked off access to the embassy building.

In London, hundreds chanted "Israel is a terrorist state" and "Free Palestine" and some waved signs calling for Israel to "end the occupation."

Meanwhile, in the U.S., supporters of Israel and of the Palestinians held competing rallies in several cities , including in New York, where rival protesters shouted at each other near U.N. headquarters amid a massive police presence.

The European Union late Monday reversed an earlier announcement by an EU commissioner that the bloc was immediately suspending aid for Palestinian authorities and instead said it would urgently review such assistance in the wake of the attacks on Israel by Hamas.

A handout photo made available by the Israeli Government Press Office shows Israeli Prime Minister Benjamin Netanyahu, left, during a situation assessment meeting in Tel Aviv, Israel, Oct. 8. EPA-Yonhap

"There will be no suspension of payments" at the moment, a terse European Commission statement said late Monday, five hours after EU Commissioner Oliver Varhelyi had said that all payments from the development program for Palestinians would be immediately suspended.

No immediate explanation for the reversal was given. The reversal on a 691 million-euro ($730 million) program capped an embarrassing day at the EU's executive at a time of extreme geopolitical sensitivities.

The Health Ministry in the Gaza Strip said Monday that more than 680 people have been killed in Israel's retaliatory strikes following an unprecedented Hamas attack.

The ministry said more than 3,700 people have been wounded.

Israel has pounded the Gaza Strip since the Saturday attack, striking hundreds of targets and leaving vast destruction.

The United Nations says intense diplomatic activity is taking place aimed at ensuring that regional and international leaders are on the same page in trying to prevent the conflict between Israel and Hamas from spreading.

U.N. Secretary-General Antonio Guterres told reporters Monday that he and U.N. Mideast envoy Tor Wennesland are engaging with key parties in the region.

Guterres has spoken to Israel's president and Jordan's king and expects to talk to the Palestinian president, Israel's prime minister, Egypt's president and Lebanon's prime minister, U.N. spokesman Stephane Dujarric said. Wennesland has been in touch with his counterparts from the United States, European Union, Qatar, Lebanon and others.

Israeli rescue service Zaka says more than 100 bodies have been recovered from a small farming community that was the scene of a hostage standoff during Hamas' attack against Israel.

The figure is part of the total 900 reportedly killed in Hamas' multi-pronged attack. Beeri, a kibbutz, had a population of about 1,000 people before the attack.

U.S. President Joe Biden plans to speak with several allies Monday regarding the situation in Israel, according to the White House.

Earlier Monday, Biden convened a meeting with top national security aides, including Secretary of State Antony Blinken, National Security Adviser Jake Sullivan, Principal Deputy National Security Adviser Jon Finer, Homeland Security Adviser Liz Sherwood-Randall and White House Chief of Staff Jeff Zients.

During that meeting, Biden urged continued coordination with Israel and other regional partners, the White House said.

The White House has called a "lid" for the day, meaning the public won't lay eyes on the U.S. president until Tuesday.

Biden has spoken at least twice with Israeli Prime Minister Benjamin Netanyahu and the White House says his top national security aides have been in regular contact with their counterparts in the region since the surprise Hamas attack on Saturday.

American oil giant Chevron Corp. said Monday that at Israel's instruction it had stopped production at its offshore Tamar natural gas rig in the Mediterranean Sea off Israel.

In a statement announcing the action, the company declined to discuss any security issues and didn't say why Israel ordered it.

The Tamar field, which went online in 2013, is believed to hold more than 300 billion cubic meters of gas. Chevron and the Israeli-American company Isramco each own around a third of Tamar, with the remainder held by smaller firms.

There have been threats previously against Israel's offshore oil rigs by militant groups in the region and Lebanon has disputed Israel's maritime boundaries.

The ongoing fighting in Israel and the Gaza Strip has sparked a jump in crude oil prices, in part over a fear of a wider regional conflict breaking out. Israel and the Palestinian territories don't produce oil. Benchmark Brent crude traded up Monday to over $87 a barrel.

Lebanon's Hezbollah group says only three of its members were killed along the border with Israel during an exchange of fire Monday.

People gather at the site where a rocket landed after being launched towards Israel from the Gaza Strip, in Ashdod, southern Israel, Oct. 9. Reuters-Yonhap

Earlier Monday, the Iran-backed Lebanese militant group had said that five members were killed during Israeli shelling in southern Lebanon.

Also Monday, the Palestinian militant group Islamic Jihad said in a statement that it sent four gunmen across Lebanon's border into Israel as part of the Hamas-led attack that started over the weekend. Israeli Defense Forces reported that its troops shot and killed several gunmen who crossed into the country from Lebanon.

The Lebanese military called on residents of border towns to "take the utmost precautions." Families in several towns in southern Lebanon started fleeing north as the Israeli shelling continued.

The top Palestinian diplomat in the U.K. says Israel is committing a war crime by cutting off water and electricity supplies to Gaza.

Husam Zomlot told a gathering of Palestinian supporters at the Labour Party's annual conference that Palestinians "need justice, not revenge."

"And what Israel is doing now is revenge, sheer vengeance."

The left-of-center Labour Party has been riven in recent years by allegations that antisemitism was allowed to spread under previous leader Jeremy Corbyn, a strong advocate of the Palestinian cause.

Labour's leadership has strongly condemned Hamas' weekend attacks, and conference delegates held a minute's silence for the victims at the conference on Monday.

The armed wing of the Palestinian militant group Hamas has warned that it will kill an Israeli hostage every time Israel's military bombs civilian targets in the Gaza Strip without warning.

Abu Obeida, the spokesman of the Qassam Brigades, said in an audio released Monday night that the threat was a response to intense air strikes by Israel on civilian areas.

"We have decided to put an end to this and as of now, we declare that any targeting of our people in their homes without prior warning will be regrettably faced with the execution of one the hostages of civilians we are holding," he said.

In a video statement Monday, Israel's foreign minister warned Hamas against harming any of the hostages who were taken from Israel and being held in Gaza. Eli Cohen said Israel was committed to bringing the hostages home "in the spirit of mutual responsibility."

"We demand Hamas not to harm any of the hostages, Cohen said. "This war crime will not be forgiven," he added.

Turkish President Recep Tayyip Erdogan held back-to-back telephone calls with Palestinian President Mahmoud Abbas and Israeli President Isaac Herzog, according to Erdogan's press office.

Erdogan and Abbas discussed the ongoing conflict between Israel and the militant group Hamas. "President Erdogan stated that Turkey is making every effort to end the conflicts in the region and ensure calm as soon as possible" a statement from his press office said.

German Chancellor Olaf Scholz says he and French President Emmanuel Macron will discuss the situation in Israel with U.S. President Joe Biden and British Prime Minister Rishi Sunak late Monday.

Scholz, who was hosting Macron at a joint German-French Cabinet retreat in Hamburg, called Hamas' attack on Israel "barbaric." But he added that Germany, France, the U.S. and the U.K. agree that there must not be a "conflagration" in the region, and "no one should further fuel terror in this situation."

Macron pledged his "full support and solidarity for Israel."

The secretary-general of the Norwegian Refugee Council, an international aid organization, warns that the Israeli government's vow to besiege and blockade the Gaza Strip would spell "utter disaster" for the more than 2 million Palestinians living in the small territory.

Jan Egeland's comments came after Israel's defense minister ordered a "complete siege" on Gaza after an unprecedented incursion by Hamas fighters into Israel early Saturday. Israel formally declared war on Sunday and has since retaliated against Hamas for the attack.

"If and when it would lead to wounded children dying in hospitals because of a lack of energy, electricity, and supplies, it could amount to war crimes," Egeland told The Associated Press in a telephone interview.

Egeland also slammed donor countries for halting humanitarian assistance to Gaza.

French police have arrested 10 people in connection with antisemitic acts that were reported since the latest fighting between Israel and Hamas militants began.

The 20 reported incidents included threats to synagogues or people frequenting Jewish stores, the interior minister said Monday.

Prosecutors have also opened 44 investigations into antisemitic hate speech online or posts glorifying terrorism in connection with the violence, according to Interior Minister Gerald Darmanin's office.

While France sees sporadic acts targeting Jews or Muslims, Darmanin said the number of antisemitic incidents since Saturday was ″dramatic.″

France has the world's largest Jewish population after Israel and the U.S.

The U.N. Palestinian refugee agency, UNRWA, says it is near maximum capacity in accommodating internally displaced people in Gaza.

The agency's director of external communications, Tamara Alrifai, said Monday that nearly 137,000 people have sheltered so far in over 70 U.N. schools around Gaza. Alrifai said the agency can host up to 150,000 people at up to 79 schools around the territory.

She added there is fuel in Gaza that could last for up to 10 days.

Arab foreign ministers plan to convene Wednesday in Cairo for a meeting on the war between Israel and the Palestinian militant groups in Gaza.

Arab League Assistant Secretary-General Hossam Zaki said the ministers would discuss Arab efforts to "stop the Israeli aggression" on Gaza.

The meeting was called by the Palestinians.

Major U.S. airlines have suspended flights to Israel after the nation declared war following a massive attack by Hamas.

American Airlines, United Airlines and Delta Air Lines suspended service as the U.S. State Department issued travel advisories for the region, citing potential for terrorism and civil unrest.

EgyptAir, Egypt's national carrier, suspended its flights to Israel until further notice, Cairo airport officials said. EgyptAir normally operates a daily flight between Cairo International Airport and Ben Gurion International Airport, just outside Tel Aviv.

Germany's Lufthansa and its subsidiaries also suspended flights to and from Tel Aviv until Saturday as fighting between Israel and Hamas continued.

An open-air electronic music festival will go down in Israeli history as the site of the country's worst civilian massacre after paramedics recovered at least 260 bodies from a field near the border with Gaza.

The Tribe of Nova festival brought together thousands of young people to dance and revel in the swirl of bass-heavy beats. Dozens of Hamas militants who had blown through Israel's heavily fortified separation fence and crossed into the country from Gaza opened fire on the Israelis.

"We were hiding and running, hiding and running, in an open field, the worst place you could possibly be in that situation," said Arik Nani from Tel Aviv, who had gone to the party to celebrate his 26th birthday. "For a country where everyone in these circles knows everyone, this is a trauma like I could never imagine."

Egypt has engaged in negotiations with Israel and Palestinian militant groups to release Palestinian women in Israel's prisons in exchange for Israeli women captured by Hamas militants, the state-owned Egyptian newspaper Al-Ahram reported Monday.

The daily paper quoted an unidentified source as saying that the negotiations were aimed at finalizing an agreement on the trade.

Palestinian militant groups have claimed to be holding over 130 people who were captured in Israel in the past two days. Hamas spokesman Abdel-Latif al-Qanoua told The Associated Press by phone that the group's fighters had captured more Israelis as recently as Monday morning.

Israeli Defense Minister Yoav Gallant said authorities would cut electricity to Gaza and block the entry of food and fuel there as part of a "complete siege" he ordered.

The announcement on Monday came as Israel's military scoured the country's south for Hamas fighters and guarded breaches in its border fence with tanks, while it pounded the Gaza Strip from the air.

Israel and Egypt have imposed various levels of blockade on Gaza since Hamas seized power from rival Palestinian forces in 2007.

The leaders of Egypt and the United Arab Emirates on Monday discussed the conflict between Israel and Palestinian militants.

Egypt's President Abdel Fattah el-Sissi and Emirati President Sheikh Mohamed bin Zayed Al Nahyan agreed on "the importance of … advancing diplomatic efforts that aim to deescalate violence, protect civilians, spare blood," a statement from the Egyptian president's office said. Such efforts should include establishing "a comprehensive, just and permanent peace," it added.

Egypt was the first Arab country to establish diplomatic ties with Israel in the 1970s, and shares borders with both Gaza and Israel. The UAE normalized ties with Tel Aviv as part of the U.S.-brokered Abraham Accords in 2020. (AP)