Russian President Vladimir Putin watches a concert at the Hanoi Opera House in Hanoi, Vietnam, Thursday. Putin returned to Moscow Friday after visiting North Korea and Vietnam. EPA-Yonhap

Fears are growing that the Korean Peninsula may become the center of a new cold war as Pyongyang and Moscow are elevating their military bond to a quasi-alliance level along with a landmark treaty which involves providing defense assistance if either nation is attacked.

In addition to strengthening their anti-West front, Russian President Vladimir Putin said in a news conference in Vietnam, Thursday (local time), that South Korea would be making "a big mistake" if it decides to supply arms to Ukraine, and Moscow would respond in a way that would be painful for Seoul.

Putin visited Vietnam after signing the comprehensive strategic partnership treaty with North Korean leader Kim Jong-un on Wednesday. He made the comments in response to South Korean Director of National Security Chang Ho-jin's remarks on Thursday that Seoul will "reconsider its earlier stance not to provide weapons to Ukraine."

"As for the supply of lethal weapons to the war zone in Ukraine, that would be a very big mistake," Putin said. "I hope that this will not happen. And if it does happen, then we will also take appropriate decisions, which are unlikely to please the current leadership of South Korea."

South Korean Director of National Security Chang Ho-jin steps up to the podium to speak at a press conference on a newly signed treaty between North Korea and Russia at the presidential office in Yongsan District, Seoul, Thursday. Joint Press Corps

An official at the South Korean presidential office said that Seoul's position on the recent developments between North Korea and Russia hinges on how Russia will approach the situation, adding that there are various options for providing weapons to Ukraine.

U.S. Department of State spokesperson Matthew Miller said Thursday (local time) that Seoul sending arms to Ukraine is "a decision for South Korea to make" but added, "We welcome any support for Ukraine in its fight against Russian aggression."

U.S. National Security Communications Advisor John Kirby also said the treaty is a concern that should be "shared by the People's Republic of China, since this agreement also seems to be in direct contrast with the statement that President Putin and President Xi made in Beijing just a month ago in which both countries called for a peaceful and diplomatic solution to the situation on the Korean Peninsula."

Russian Ambassador to Seoul Georgy Zinoviev enters the South Korean Ministry of Foreign Affairs in central Seoul, Friday, as he was called in to receive Seoul's protests over a treaty between North Korea and Russia earlier this week. Yonhap

In response to Putin's remarks, South Korea's Ministry of Foreign Affairs on Friday called in Russian Ambassador to Seoul Georgy Zinoviev to lodge its protest. Seoul stressed to the summoned diplomat that the treaty is a violation of United Nations Security Council resolutions and threatens not only bilateral relations between Seoul and Moscow but also regional stability.

Ambassador Zinoviev, for his part, stressed that any attempts to threaten Russia cannot be accepted, according to the Russian Embassy in Seoul.

As a counter to the Kim-Putin treaty, South Korean Foreign Minister Cho Tae-yul had phone conversations with U.S. Secretary of State Antony Blinken and Japanese Foreign Minister Yoko Kamikawa.

In the calls, the ministers slammed the treaty for posing a significant threat to the security of the region, and discussed ways for strengthening trilateral security cooperation.

As South Korea, the U.S., and Japan pledged trilateral security cooperation to counter the enhanced relationship between North Korea and Russia, the rising tensions between the two Koreas are increasingly linked to Russia's invasion of Ukraine. This development stokes fears that a new cold war may unfold on the Korean Peninsula.

"In the wake of Russia's invasion of Ukraine, the confrontation between values and political systems has worsened," Asan Institute for Policy Studies Principal Fellow Cha Du-hyeogn said.

"Russia is seeking to form a coalition with countries sharing similar traits, and North Korea's Kim is taking advantage of this situation, showing that this is a natural realignment amid the ongoing clash of values. Simply put, regardless of whether South Korea supports Ukraine or not, Russia would have approached North Korea."