Washington earlier this month designated the Wagner group as a "transnational criminal organization," citing its weapons dealings with Pyongyang in violation of UN Security Council resolutions.

The White House showed US intelligence photographs of Russian rail cars entering North Korea, picking up a load of infantry rockets and missiles, and returning to Russia, according to national security spokesman John Kirby.

In a statement carried by the official Korean Central News Agency, a senior North Korean official rejected the accusations, warning that the US will face a "really undesirable result" if it persists in spreading the "self-made rumour".

"Trying to tarnish the image of (North Korea) by fabricating a non-existent thing is a grave provocation that can never be allowed and that cannot but trigger its reaction," said Kwon Jong Gun, director general of the Department of US Affairs.

He also called it "a foolish attempt to justify its offer of weapons to Ukraine".

Earlier this week, US President Joe Biden promised 31 Abrams tanks, one of the most powerful and sophisticated weapons in the US army, to help Kyiv fight off Moscow's invasion.

The move drew a rebuke Friday from Kim Yo Jong, the powerful sister of North Korean leader Kim Jong Un, who accused Washington of "further crossing the red line" by sending the tanks into Ukraine.

Along with China, Russia is one of the North's few international friends and has previously come to the regime's aid.

Other than Syria and Russia, North Korea is the only country to recognise the independence of Luhansk and Donetsk, two Russian-backed separatist regions in eastern Ukraine.

Russia, one of the five permanent members of the UN Security Council, has long held the line against increasing pressure on nuclear-armed North Korea, even asking for relief from international sanctions for humanitarian reasons.

Kim Jong Un declared North Korea an "irreversible" nuclear state in September, and the country conducted sanctions-busting weapons tests nearly every month last year -- including firing its most advanced intercontinental ballistic missile.