Russian President Vladimir Putin thanked North Korea for supporting his actions in Ukraine and said their countries will co-operate closely to overcome U.S.-led sanctions, as he headed to Pyongyang on Tuesday for a summit with North Korean leader Kim Jong-un.

Putin's comments appeared in an op-ed piece in North Korean state media hours before he was expected to arrive for a two-day visit as the countries deepen their alignment in the face of separate, intensifying confrontations with Washington.

Putin, who will be making his first trip to North Korea since his first year as Russian president in 2000, said he highly appreciates its firm support of his invasion of Ukraine. He said the countries would continue to "resolutely oppose" what he described as Western ambitions "to hinder the establishment of a multipolarized world order based on mutual respect for justice."

Putin also said Russia and North Korea will develop trade and payment systems "that are not controlled by the West" and jointly oppose sanctions against the countries, which he described as "unilateral and illegal restrictive measures."

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Putin said the countries will also expand co-operation in tourism, culture and education.

North Korea is under heavy UN Security Council economic sanctions over its nuclear weapons and missile programs, while Russia is also grappling with sanctions by the United States and its Western partners over its aggression in Ukraine.

Putin has limited his foreign travel since sending troops in to invade Ukraine in 2022, a decision that led to an arrest warrant issued by the International Criminal Court. After North Korea, Putin will visit Vietnam, which is also not a signatory to the jurisdiction of the ICC.

Putin's visit comes amid growing concerns about an arms arrangement in which Pyongyang provides Moscow with badly needed munitions to fuel Russia's war in Ukraine in exchange for economic assistance and technology transfers that would enhance the threat posed by Kim's nuclear weapons and missile program.

In Pyongyang, streets were decorated with Putin's portraits and Russian flags. A banner hung on a building said: "We warmly welcome the President of the Russian Federation."

Accusations, denials of weapon use in Ukraine

John Kirby, spokesperson of the U.S. National Security Council, said the deepening relationship between Moscow and Pyongyang is concerning, "not just because of the impacts it's going to have on the Ukrainian people, because we know North Korean ballistic missiles are still being used to hit Ukrainian targets, but because there could be some reciprocity here that could affect security on the Korean Peninsula."

"We haven't seen the parameters of all of that right now, certainly haven't seen it come to fruition. But we're certainly going to be watching that very, very closely," he said.

Military, economic and other exchanges between North Korea and Russia have sharply increased since Kim visited Russia in September for a meeting with Putin, their first since 2019.

U.S. and South Korean officials have accused the North of providing Russia with artillery, missiles and other military equipment to help prolong the war in Ukraine, possibly in return for key military technologies and aid. Both Pyongyang and Moscow have denied accusations about North Korean weapons transfers.

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

In March, a Russian veto at the United Nations ended monitoring of UN sanctions against North Korea over its nuclear program, prompting Western accusations that Moscow is seeking to avoid scrutiny as it buys weapons from Pyongyang for use in Ukraine. 

Lim Soosuk, spokesperson of South Korea's Foreign Ministry, said Seoul has been stressing to Moscow that any co-operation between Russia and North Korea must not "proceed in a direction that violates UN Security Council resolutions or undermines peace and stability in the region."

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Tensions on the Korean Peninsula are at their highest point in years, with the pace of both Kim's weapons tests and the combined military exercises between the United States, South Korea and Japan intensifying in a cycle of tit-for-tat. The Koreas have also engaged in a Cold War-style psychological warfare that involved North Korea dropping tons of trash on the South with balloons and the South broadcasting anti-Korean propaganda broadcasts with its loudspeakers.

South Korea's military said soldiers fired warning shots to repel North Korean soldiers who temporarily crossed the rivals' land border Tuesday for the second time this month. The South's military said North Korea has been increasing construction activity in front-line border areas, such as installing suspected anti-tank barriers, reinforcing roads and planting land mines.