Ukraine's president accused North Korea on Thursday of deploying officers alongside Russia and preparing to send thousands of troops to help Moscow's war effort, although NATO's chief said there was no evidence of Pyongyang's presence at this stage.

Western countries have long accused North Korea of sending weapons to Russia, and in recent days President Volodymyr Zelenskyy has said it was also sending personnel, a significant escalation of foreign assistance for Moscow's invasion.

"We have information from our intelligence that ... some officers of the North Korean army are already on Ukrainian territory temporarily occupied by the Russian enemies. So they joined the Russian army," Zelenskyy told a press conference in Brussels.

He said he could not give the exact number that were on the ground.

"We know about 10,000 soldiers of North Korea, that they are preparing to send to fight against us," he said, describing it as a "first step to the World War."

Ukraine's Western allies have yet to confirm Kyiv's assertion that Pyongyang is sending troops, though they say they are studying it.

NATO Secretary-General Mark Rutte said in a press conference alongside the Ukrainian leader on Thursday that "we have no evidence that North Korean soldiers are involved in the fight."

"We do know that North Korea is supporting Russia in many ways – by weapon supplies, technological supplies, innovation – to support them in the war effort, and that is highly worrying," he said.

White House National Security Council spokesperson Sean Savett said on Tuesday any North Korean troop involvement in Ukraine, if true, would mark a significant increase in the Moscow-Pyongyang defense relationship.

Zelenskyy said Russia needs to fill a gap in mobilization and cover its own personnel losses.

Ukraine and the West say hundreds of thousands of Russians have been killed or wounded fighting in Ukraine, while Ukraine's own military losses, a closely guarded secret, are also high but smaller. Moscow's forces have advanced over the past several months in Ukraine's east.

Ukraine has called on allies to respond firmly to North Korean aid for Russia, including by imposing new sanctions and further isolating Pyongyang.