Russian President Vladimir Putin has signed a mutual defence agreement with North Korea, whose leader Kim Jong Un offered his "full support" on Ukraine.
The pledge of military cooperation was part of a strategic treaty signed during a summit in Pyongyang, where Mr Putin was making his first visit in 24 years.
"It is really a breakthrough document," he said at a news conference in the North Korean capital, adding the deal provided, "among other things, for mutual assistance in case of aggression against one of the parties", Russian news agencies said.
The two countries have been allies since North Korea's founding after World War II and have drawn even closer since Russia's 2022 invasion of Ukraine.
The United States and its allies have accused North Korea of providing ammunition and missiles to Russia for the war, and the treaty is certain to fuel concerns of more deliveries.
President Putin also said that Russia "does not rule out military-technical cooperation with the DPRK in connection with the treaty", referring to North Korea by its official name.
Mr Kim described the Russian leader as the "dearest friend of the Korean people" and said his country "expresses full support and solidarity to the Russian government" over the war in Ukraine, which has triggered rafts of sanctions on Moscow.
Mr Putin, in turn, thanked the North Korean dictator - whose country has been under a United Nations sanctions regime since 2006 over its weapons programmes - saying Moscow appreciated the "consistent and unwavering" support.
The two heavily sanctioned countries would not tolerate Western "blackmail", President Putin said, calling for a review of UN sanctions on North Korea.
"The indefinite restrictive regime inspired by the US and its allies at the UN Security Council towards the DPRK should be reviewed.
"Today, we are fighting together against the hegemonism and neo-colonial practices of the United States and its satellites," he said.
President Putin was greeted by Mr Kim on a red carpet at Pyongyang airport, where the pair embraced and smiled.
They then attended a welcoming ceremony in Kim Il Sung Square, featuring a military band and mass synchronised dancing, after which the Russian leader invited his host to visit Moscow.
Mr Putin gifted Mr Kim a luxury car by Russian carmaker Aurus, and took him for a drive, according to the state-run RIA Novosti news agency.
The two also visited an Orthodox Church in Pyongyang, it added.
The summit, which included a lengthy one-on-one chat between the leaders, was their second meeting in a year.
Mr Kim took his bulletproof train to Russia's Far East in September for a summit with President Putin at a spaceport.
The North Korean leader said the two countries' ties had risen "to a new high of alliance".
"It is greatly satisfying to conclude a great treaty that befits a changed international situation and the strategic nature of new DPRK-Russia relations," he said.
Mr Kim added that the agreement treaty "fully contributes to maintaining peace and stability in the region".
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The US State Department said that "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".
"As we have said before, we don't believe any country should give Mr. Putin a platform to promote his war of aggression against Ukraine. Russia is blatantly violating the UN Charter and working to undermine the international system." a spokesperson added.
Mykhailo Podolyak, a senior aide to Ukraine's President Volodymyr Zelensky, said that North Korea is abetting Russia's "mass murder of Ukrainians", and called for greater international isolation of both countries.
Professor of Korean Studies at the University of Oslo Vladimir Tikhonov said that Russia would now "largely sabotage the sanctions regime around North Korea, in deed if not in word".
The new mutual support clause is "a reminder to Americans that Russia may complicate their lives if they support Ukraine too enthusiastically", he said, pointing to the roughly 28,000 US troops based in South Korea, a key regional security ally of Washington.
The two Koreas have remained technically at war since their 1950-53 conflict and the border dividing them is one of the most heavily fortified in the world.
This week's visit was a way for President Putin to thank the North "for acting as an 'arsenal for autocracy' in support of his illegal invasion of Ukraine", said Leif-Eric Easley, a professor at Ewha University in Seoul.
It was also part of Russia's drive to secure "strategic space" in Northeast Asia to counter US influence in the region, Kim Sung-bae, a research fellow at the Seoul-based Institute for National Security Strategy, said.
"This intention is further evidenced by Mr Putin's visit to Vietnam," he said.
Russia's president arrived in Vietnam tonight for talks with the country's Communist leaders, according to Russian news agencies.
The agencies said that his aircraft touched down in Hanoi on the final stop of his two-nation tour of Asia.
Mr Putin praised Vietnam for its "balanced" stance on the Ukraine war and listed progress on payments, energy and trade in an opinion piece published in the newspaper of Vietnam's Communist Party.
He applauded the country for supporting "a pragmatic way to solve the crisis" in Ukraine, in the Nhan Dan publication.
