The United States warned on Monday that North Korean soldiers were moving toward Russia’s western Kursk region, which Ukraine invaded in August, as Ukrainian forces braced for what they said could be imminent assaults involving the new troops.

The Pentagon said North Korea had now sent about 10,000 soldiers to train in eastern Russia, with many moving toward the battlefield in the Kursk region. The NATO secretary general, Mark Rutte, on Monday confirmed that North Korean troops had been deployed in Kursk, saying it represented “a dangerous expansion” of the war.

Ukrainian and American officials said last week that several thousand North Korean troops had already arrived in the Kursk area. Military experts say that is too small a number to affect the overall situation on the broader battlefield, where both sides have deployed hundreds of thousands of soldiers, but potentially enough to help Moscow reclaim its territory in the Kursk region.

“As their numbers grow, I expect their impact to be seen by the progress of a steady Russian counterattack,” said John Foreman, a former British defense attaché in Moscow and Kyiv.

A Ukrainian official said Monday that the North Korean soldiers had been deployed to camps and were living in temporary barracks between 25 to 40 miles from the Ukrainian border. The official, who spoke on the condition of anonymity to discuss internal military information, said the North Koreans had not yet joined the fighting.

It is unclear how exactly North Korean troops will support Russia’s counterattack in the Kursk region. Analysts say the soldiers could be used in direct attacks or to guard areas behind the combat zone, thus freeing up Russian troops for assaults, but their effectiveness in battle is untested and could be hampered by coordination issues with the Russians.