Some N Korean officers already deployed with Russians in Ukraine: Zelenskyy

Thousands of North Korean troops are being prepared to fight on Russia's behalf in Ukraine and some North Korean officers have already been deployed there, Ukrainian President Volodymyr Zelenskyy has claimed.

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

"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 news conference in Brussels on Thursday.

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

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

The North Korean troops being prepared to fight in Ukraine include land forces and "other technical personnel", Zelenskyy said.

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

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 defence relationship.

Zelenskyy said Russia needs to fill the gap in mobilisation 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.