North Korean soldiers “disguised” as Russian fighters from Siberia have clashed with Ukrainian troops, Kiev’s defense minister said, although the number of casualties among Pyongyang’s forces is still unclear.

North Korean troops deployed alongside Moscow’s own forces were dressed to look like Russian recruits from the Buryatia region of eastern Siberia, Rustem Umerov told South Korean broadcaster KBS.

Ukrainian, South Korean and Western intelligence have said in recent weeks that North Korea was sending between 10,000 and 12,000 soldiers in Russia to support Moscow’s war effort against Kiev.

Umerov said there had been “small-scale clashes” so far between Ukrainian and North Korean troops, but that Ukraine could not yet verify how many casualties North Korea had suffered or how many soldiers had become prisoners of war.

An unnamed Ukrainian official said The New York Times in an article published Tuesday that engagements involving North Korean troops have been limited, likely intended to test Ukraine’s lines for weaknesses.

Pyongyang’s troops joined Russia’s 810th Separate Naval Infantry Brigade, the official said. US think tank, Institute for the Study of War (ISW), said on Tuesday that Russia is likely trying to integrate North Korean fighters into Russia’s military structure rather than having “separate North Korean units fighting under Russian command.”

An American official told him Times a significant number of North Korean troops were killed, but did not specify more.

“We have identified contacts with North Korean forces, but we expect more engagements in the coming weeks and will review and review accordingly,” Umerov added.

He said up to 15,000 troops could end up supporting Russian forces in Russia’s southern Kursk region, where Moscow has been fighting to wrest Ukrainian control of a section of Russian territory near the border since early August.

The Pentagon said on Monday that 10,000 North Korean troops were in Kursk. At the end of October, the US Secretary of Defense Lloyd Austin said the US had not yet detected North Korean troops involved in the fighting, but that they would “join the fight against Ukraine in the coming days”.

Many Western countries have denounced the arrival of North Korean troops to fight in Europe’s biggest land war since World War II as a dangerous escalation in the grinding and bloody conflict.

“The first battles with North Korean soldiers open a new page of instability in the world,” said the Ukrainian president Volodymyr Zelensky he said in his speech on Tuesday evening. “We must do everything to make this Russian step to expand the war — to really escalate it — to make this step a failure.”

Andriy Kovalenko, head of countering disinformation at Ukraine’s National Security and Defense Council, said Monday that the first North Korean troops “were already under fire” at Kursk.

The next day, Kovalenko said the North Koreans were “in Russian military uniform” alongside Russian units in Kursk, learning how to use different types of drones against Ukrainian forces. The US has previously said that Moscow supplied North Korean troops with Russian military uniforms.

South Korea’s National Intelligence Service spy agency said last month that an initial batch of 1,500 North Korean fighters traveled to Russia and were equipped with Russian army uniforms, Russian-made weapons and fake documents claiming the fighters were residents of the Siberian regions.

“We expect more engagements in the coming weeks,” Umerov said.

Pyongyang has supplied Moscow with a significant number of missiles and millions of shells. Its support, the head of military intelligence in Kiev previously said, makes North Korea the most formidable ally of Russia for Ukraine to face. Kiev has persistently targeted ammunition depots storing North Korean munitions in recent months.