Russia's President Vladimir Putin, left, and North Korea's leader Kim Jong-un attend a state reception in Pyongyang, North Korea, Wednesday. Reuters-Yonhap

South Korea strongly condemned a new treaty signed by North Korea and Russia, which involves providing mutual defense assistance “without delay” in the event of an attack on either nation, calling it a violation of U.N. Security Council (UNSC) resolutions, Thursday.

The presidential office warned that it may consider supplying arms to Ukraine in response to the growing military cooperation between Moscow and Pyongyang.

The condemnation came a day after Russian President Vladimir Putin and North Korean leader Kim Jong-un signed a comprehensive strategic partnership treaty during their summit in Pyongyang.

“We express grave concern and condemn the signing of the comprehensive strategic partnership agreement between North Korea and Russia. Any cooperation that directly or indirectly supports the military enhancement of North Korea is a clear violation of UNSC resolutions and is subject to sanctions monitoring by the international community,” National Security Adviser Chang Ho-jin said at a press briefing.

“The fact that Russia, once a supporter of U.N. sanctions against North Korea as a permanent member of the UNSC, is now posing a threat to our security by violating these sanctions, will negatively impact our relations with Russia.”

Chang added that South Korea will sternly respond to any actions that threaten its security by strengthening the Seoul-Washington alliance on extended deterrence and enhancing security cooperation between South Korea, the U.S., and Japan.

A presidential office official said South Korea will reconsider its earlier stance not to provide weapons to Ukraine to help the country fight off Russia's invasion, without providing further details. The official added that Seoul will take specific measures depending on Russia's response.

South Korea also imposed additional sanctions on four Russian ships, five organizations, and eight individuals for various activities, including allegedly engaging in transactions of military supplies between Moscow and Pyongyang.

Currently, there are 1,159 items subject to export controls to Moscow following the Russia-Ukraine war. Seoul will add 243 new items, bringing the total number of sanctioned items to 1,402.

National Security Adviser Chang Ho-jin speaks during a press briefing at the presidential office in Seoul, Thursday. Yonhap

The international community also voiced a wave of criticism against the actions taken by Russia and North Korea.

The U.S. expressed great concern over the close ties between Russia and North Korea. "Deepening cooperation between Russia and the DPRK is a trend that should be of great concern to anyone interested in maintaining peace and stability on the Korean Peninsula, upholding the global non-proliferation regime, abiding by U.N. Security Council resolutions, and supporting the people of Ukraine as they defend their freedom and independence against Russia's brutal invasion," a state department spokesperson told Yonhap News Agency.

Japanese Chief Cabinet Secretary Yoshimasa Hayashi vowed to cooperate with South Korea, the U.S. and the international community for the complete implementation of U.N. resolutions.

“Russia's procurement of weapons and ammunition from North Korea, including ballistic missiles, for use in Ukraine is totally unacceptable,” Hayashi said.

“We will continue to cooperate with the international community, push for the full implementation of relevant UNSC resolutions and demand the complete elimination of North Korea's nuclear weapons programs."

Against this backdrop, concerns are mounting that the strengthened relationship between North Korea and Russia could undermine U.N. sanctions as both countries pledged to resist the U.S.-led measures.

Some analysts view their agreement on military assistance in the event of an attack on either nation, as comparable to a clause in their 1961 treaty, which mandated automatic military intervention in case of an armed invasion.

During the summit, Putin emphasized that Russia and North Korea would jointly oppose sanctions imposed on them by the U.S. and its allies. He added that U.N. sanctions against Pyongyang should be "reviewed," following the signing of a comprehensive strategic partnership treaty between the two leaders.

"I am noting that the indefinite restrictive regime inspired by the U.S. and its allies at the UNSC toward North Korea should be reviewed," Putin was quoted as saying by Russian media during a joint press conference after the summit with Kim. “We oppose the practice of applying politically motivated sanctions and restrictions. Such illegitimate actions only undermine the global economic and political system.”

Russia, which previously backed U.N. sanctions that explicitly prohibited North Korea from exporting weapons, now describes them as "politically motivated." Additionally, Russia mentioned the possibility of exploring "military-technical cooperation" with North Korea, potentially involving the import of more munitions and missiles from Pyongyang. However, they have denied any involvement in weapons transfers from North Korea for use in the conflict in Ukraine.

The treaty came as the North and Russia have been bolstering military and security ties. The U.S. has accused North Korea of supplying “dozens of ballistic missiles and more than 11,000 containers of munitions to Russia” for use in Ukraine in exchange for aid and more advanced military technologies.

On Wednesday, Kim flaunted his Mercedes Benz limousine at a welcoming ceremony at Kim Il Sung Square, while Putin gave Kim a new Aurus limousine as a gift, demonstrating that international sanctions have distinct limits.

Russian President Vladimir Putin and North Korea's leader Kim Jong-un walk during their meeting in Pyongyang, Wednesday. AFP-Yonhap

Experts feared that the growing Russia-North Korea cooperation might neutralize U.N. sanctions.

“Russia not only denounced sanctions imposed on the North Korean regime but on itself too," said Park Won-gon, professor of North Korean studies at Ewha Womans University.

"His public remark came under the notion that Russia would be met with criticism as a permanent member of the UNSC. In April, the Russian veto at the UNSC blocked the renewal of a panel of experts tasked with stopping North Korea from conducting nuclear tests or launching ballistic missiles. They underscored that the U.N. has been ineffective in achieving its objectives.”

Lim Eul-chul, a professor of North Korean studies at Kyungnam University, said extensive cooperation between Russia and North Korea indicates a strong determination by both countries to disregard restrictions imposed by the United Nations.

“Apart from strengthening military and security ties, cooperation in a wide range of fields between Russia and North Korea have been confirmed, including food, energy, information and technology, transport, health care, space, science and artificial intelligence. Most of these are virtually impossible to implement if Russia complies with the UNSC. So this can be interpreted as showing the world that they are willing to pursue them regardless of sanctions in the future,” Lim said.