Russia and North Korea have boosted their political and military alliance in the course of the conflict, but the deployment of Pyongyang's troops into combat against Kyiv's forces would mark a significant escalation.

"We believe that the DPRK has sent around 10,000 soldiers in total to train in eastern Russia that will probably augment Russian forces near Ukraine over the next several weeks," Deputy Pentagon Press Secretary Sabrina Singh told journalists, using an abbreviation for North Korea's official name.

"A portion of those soldiers have already moved closer to Ukraine, and we are increasingly concerned that Russia intends to use these soldiers in combat or to support combat operations against Ukrainian forces in Russia's Kursk Oblast," Singh said.

The number of North Korean troops in Russia could grow further "as Putin's desperation continues to rise," she said of Russia's president, whose forces have reportedly suffered hundreds of thousands of casualties in Ukraine.

US National Security Council spokesman John Kirby previously put the number of North Korean troops in Russia at more than 3,000 on October 23, warning that they would become "legitimate military targets" if they fight against Ukraine.

NATO chief Mark Rutte on Monday decried the deployment of North Korean troops to bolster Russia's war effort in Ukraine as a dangerous expansion of the conflict that signaled Putin's "growing desperation."

Experts have said that in return for the troops, North Korea is likely aiming to acquire military technology, ranging from surveillance satellites to submarines, plus possible security guarantees from Moscow.

North Korea and Russia are under UN sanctions -- Pyongyang for its nuclear weapons program, and Moscow for the Ukraine war.