North Korea has not sent troops to Russia to help Moscow fight Ukraine, one of its United Nations representatives said on Monday, dismissing Seoul's claims as a "groundless rumour".

Seoul's spy agency had said that Pyongyang sent a "large-scale" troop deployment to help its ally, claiming that 1,500 special forces were already training in Russia's Far East and ready to head soon for the frontlines of the Ukraine war.

"As for the so-called military cooperation with Russia, my delegation does not feel any need for comment on such groundless stereotyped rumours," a North Korean representative said at a committee meeting during the UN General Assembly.

Seoul's claims were "aimed at smearing the image of the DPRK and undermining the legitimate, friendly and cooperative relations between two sovereign states," the representative told the meeting, held late on Monday in New York.

North Korean state media have not commented on the purported troop deployment, and Russia has also not confirmed the troop deployment, but defended its military cooperation with the North.

After Seoul summoned Moscow's ambassador in South Korea to complain, the envoy "stressed that cooperation between Russia and North Korea... is not directed against the interests of South Korea's security".

Neither Nato nor the United States have confirmed the deployment, but both have cast it as a potentially dangerous escalation in the long-running Ukraine conflict.

"We have seen reports the DPRK has sent forces and is preparing to send additional soldiers to Ukraine to fight alongside Russia," Robert Wood, US ambassador to the United Nations, told the Security Council.

"If true, this marks a dangerous and highly concerning development and an obvious deepening of the DPRK-Russia military relationship," Wood said. (AFP)