Ukrainian special forces fighting in Russia’s western Kursk region have told the BBC they have not seen any North Korean troops there for the past three weeks.
A spokesman said it was likely they had pulled out after suffering heavy losses.
Last week, Western officials told the BBC that, out of some 11,000 troops sent from North Korea to fight for Russia, 1,000 had been killed in just three months.
North Korea and Russia have not commented.
On Friday, the Ukrainian special forces spokesman told the BBC he was only referring to areas in the Kursk region where his forces were fighting.
The spokesman did not say how long that front line was.
And while this is not the full picture, it does suggest significant North Korean casualties.
Separately, the New York Times also reported that the North Koreans had been pulled off the front lines.
The newspaper quoted US officials as saying the withdrawal may not be a permanent one, and the soldiers could return after receiving additional training or after the Russians come up with new ways of deploying them to avoid such heavy casualties.
Reports attributed to South Korean intelligence say the North Koreans are unprepared for the realities of modern warfare, and are especially vulnerable to being targeted by Ukrainian drones.
Russian President Vladimir Putin and North Korea’s leader Kim Jong Un have in recent months deepened bilateral ties, signing a security and defence treaty.
Pyongyang’s assistance to Moscow now also extends to large amounts of ammunition and weapons.
Last August, elite Ukrainian troops launched a lightning offensive in Kursk, seizing more than 1,000 sq km (386 sq miles) of Russian territory.
Since then, Russian forces have managed to retake a sizeable chunk of that region.
Kyiv’s surprise incursion was aimed at changing the dynamics of the war.
It was initially hoped the operation would relieve pressure on other parts of the more than 1,000km-long (620 miles) frontline, particularly in eastern Ukraine.
Moscow is continuing its relentless – albeit slow – advance in the east, seizing a number of settlements in the Donetsk region in recent weeks.
Now Kyiv is looking to hold on to the land it occupies in Kursk as leverage for any possible ceasefire or peace negotiations with Moscow.
Russia launched a full-scale invasion of Ukraine in February 2022.
