SEOUL, South Korea — Russian President Vladimir Putin and North Korean leader Kim Jong Un signed an agreement Wednesday that pledges mutual aid if either country faces “aggression,” a strategic pact that comes as both face escalating standoffs with the West.

Details of the deal were not immediately clear, but it could mark the strongest connection between Moscow and Pyongyang since the end of the Cold War. Both leaders described it as a major upgrade of their relations, covering security, trade, investment, cultural and humanitarian ties.

The summit came as Putin visited North Korea for the first time in 24 years and the U.S. and its allies expressed growing concerns over a possible arms arrangement in which Pyongyang provides Moscow with badly needed munitions for its war in Ukraine, in exchange for economic assistance and technology transfers that could enhance the threat posed by Kim’s nuclear weapons and missile program.

Historical Significance of the Agreement

Kim said the two countries had a “fiery friendship,” and that the deal was their “strongest ever treaty,” putting the relationship at the level of an alliance. He vowed full support for Russia’s war in Ukraine.

Putin called it a “breakthrough document” reflecting shared desires to move relations to a higher level.

North Korea and the former Soviet Union signed a treaty in 1961, which experts say necessitated Moscow’s military intervention if the North came under attack. The deal was discarded after the collapse of the USSR, replaced by one in 2000 that offered weaker security assurances. It wasn’t immediately clear if the new deal provides a similar level of protection as the 1961 treaty.

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Kim met Putin at the airport, where the two shook hands, hugged twice and rode together in a limousine. The huge motorcade rolled through the capital’s brightly lit streets, where buildings were decorated with giant Russian flags and portraits of Putin.

After spending the night at a state guest house, Putin was welcomed Wednesday morning in a ceremony at the city’s main square, filled with what appeared to be tens of thousands of spectators, including children with balloons and people in coordinated T-shirts of the red, white and blue national colors of both countries. Crowds lining the streets chanted “Welcome Putin,” and waved flowers and flags.

Putin and Kim saluted an honor guard and walked across a red carpet. Kim introduced key members of his leadership including Foreign Minister Choe Son Hui; top aide and ruling party secretary Jo Yong Won; and the leader’s powerful sister, Kim Yo Jong.

At their talks, Putin thanked Kim for North Korea’s support in Ukraine, part of what he said was a “fight against the imperialist hegemonistic policies of the U.S. and its satellites against the Russian Federation.”

Putin praised ties that he traced to the Soviet army fighting the Japanese military on the Korean Peninsula at the end of World War II, and Moscow’s support for Pyongyang during the Korean War.

What kind of support was pledged in the agreement was not spelled out. Kim has used similar language before, consistently saying North Korea supports what he describes as a just action to protect Russia’s interests and blaming the crisis on the West’s “hegemonic policy.”

North Korea is under heavy U.N. Security Council sanctions over its weapons program, while Russia also faces sanctions by the U.S. and its Western partners over its invasion of Ukraine.

International Reactions and Implications

U.S. and South Korean officials accuse the North of providing Russia with artillery, missiles and other military equipment for use in Ukraine, possibly in return for key military technologies and aid. On Tuesday, a U.S. State Department spokesman said that in recent months, Washington has seen North Korea “unlawfully transfer dozens of ballistic missiles and over 11,000 containers of munitions to aid Russia’s war effort.”

Both Pyongyang and Moscow deny accusations of weapons transfers, which would violate multiple U.N. Security Council sanctions that Russia previously endorsed.

Along with China, Russia has provided political cover for Kim’s efforts to advance his nuclear arsenal, repeatedly blocking U.S.-led efforts to impose fresh U.N. sanctions on the North over its weapons tests.

In March, a Russian veto in the Security Council ended monitoring of U.N. sanctions against North Korea over its nuclear program, prompting Western accusations that Moscow is seeking to avoid scrutiny as it buys weapons from Pyongyang.

Putin’s foreign affairs adviser Yuri Ushakov told reporters in Pyongyang the leaders exchanged gifts after the talks. Putin presented Kim with a Russian-made Aurus limousine and other gifts, including a tea set and a naval officer’s dagger. Ushakov said Kim’s presents to Putin included artwork depicting the Russian leader.

Later, Putin and Kim attended a concert featuring marching soldiers, weapons-throwing, dancing and patriotic songs. Putin clapped and spoke to Kim through a translator, saying something that made both laugh.

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At a dinner before Putin left for Vietnam, he cited a proverb that said “a close neighbor is better than a distant relative,” while Kim toasted the “immortality of the invincible DPRK-Russia relations that are the envy of the world.”

Earlier, Putin said the partnership included cooperation in political, trade, investment, cultural and humanitarian fields, in addition to security. He added that Russia would not rule out developing military-technical cooperation with North Korea.

The Kremlin’s website said they also signed an agreement to build a road bridge on their border, and another on cooperation in health care, medical education and science.

In Washington, U.S. Secretary of State Antony Blinken said Putin’s visit to North Korea illustrates how Russia tries, “in desperation, to develop and to strengthen relations with countries that can provide it with what it needs to continue the war of aggression that it started against Ukraine.”

Koo Byoungsam, spokesperson of South Korea’s Unification Ministry, which handles inter-Korean affairs, said the Seoul government was still interpreting the results of the summit, including what Russia’s response might be if the North comes under attack.

China is North Korea’s biggest ally and economic lifeline, accounting for most of the country’s trade. Chinese Foreign Ministry spokesperson Lin Jian said high-level exchanges between Moscow and Pyongyang are “bilateral arrangements between two sovereign states,” without giving a specific assessment of the agreements.

Sam Greene of the Center for European Policy Analysis said Putin’s trip to Pyongyang is an indication of how beholden he is to some other countries since invading Ukraine. Previously, “it was always the North Koreans coming to Russia. It wasn’t the other way around,” he said.

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The trip is a good way to make “the West nervous” by demonstrating Moscow has interests and clout beyond Ukraine, Greene added.

The North could also seek to increase labor exports to Russia and other activities to get foreign currency in defiance of U.N. sanctions, according to the Institute for National Security Strategy, a think tank run by South Korea’s main spy agency. There will likely be talks about expanding cooperation in agriculture, fisheries and mining and further promoting Russian tourism to North Korea, the institute said.

Tensions on the Korean Peninsula are at their highest point in years, with the pace of both Kim’s weapons tests and combined military exercises involving the U.S., South Korea and Japan intensifying in a tit-for-tat cycle.

The Koreas also have engaged in Cold War-style psychological warfare that involved North Korea dropping tons of trash on the South with balloons, and the South broadcasting anti-North Korean propaganda with its loudspeakers.