A top North Korean military official on Monday criticised the United States over its expanding military assistance to Ukraine, reaffirming the reclusive state's support for Moscow in the Ukraine war, according to state media KCNA.

Washington and Seoul have been increasingly alarmed by deepening military cooperation between Russia and the North, and have accused them of violating international laws by trading in arms for Russia to use against Ukraine. Moscow and Pyongyang have denied any arms transfer.

A pact signed by Russia's Vladimir Putin and North Korean leader Kim Jong Un during Putin's visit to Pyongyang last week commits each side to provide immediate military assistance to the other in the event of armed aggression against either one of them.

Putin on Monday thanked Kim for his hospitality during the trip which brought ties to an unprecedented level, the Kremlin said on Monday.

Analysts say the pact would lay the framework for arms trade between the two countries and facilitate their anti-U.S. and anti-West coalition.

Pak Jong Chon, one of North Korea's top military officials, said Russia has the "right to opt for any kind of retaliatory strike" in a statement carried by KCNA on Monday, adding if Washington kept pushing Ukraine to a "proxy war" against Russia, it could provoke a stronger response from Moscow, and a "new world war".

He referred to comments by the Pentagon last week that Ukrainian forces can use U.S.-supplied weapons to strike Russian forces anywhere across the border into Russia.

Senior officials of South Korea, the U.S. and Japan condemned "in the strongest possible terms" deepening military cooperation between North Korea and Russia in a joint statement released by Seoul's foreign ministry on Monday.

Russia may have received about 1.6 million artillery shells from North Korea from August to January, the Washington Post reported on Saturday, analysing data from a U.S. security nonprofit C4ADS that shows 74,000 metric tons of explosives moved from Russia's far east ports to other sites mainly along the borders near Ukraine.

Putin's mutual defence agreement with North Korea has the potential to create friction with China, which has long been the isolated state's main ally, the top U.S. military officer said on Sunday.

North Korea plans to send construction and engineering forces to Russia-occupied territories of Ukraine as early as next month for rebuilding work, a South Korean cable TV network TV Chosun reported earlier, citing a South Korean government official.

Those forces, working overseas under the disguise of construction workers to earn hard currency for the regime, would be moved from China to those Russia-held regions, the network said.

South Korea's foreign ministry was not immediately available for comment on the TV Chosun reports.