The U.S. Department of Defense (Pentagon) stated on Monday that Ukraine will not face any new restrictions on the use of American weapons against North Korean forces if the latter fights Ukrainian troops, while estimating that 10,000 North Korean soldiers have been deployed in eastern Russia.

Pentagon spokesperson Sabrina Singh said at a press conference on Monday that North Korea has sent 10,000 soldiers to Russia for training and fighting in Ukraine over the "coming weeks."

She added that "some of these soldiers have already moved to a location closer to Ukraine, and we are increasingly concerned that Russia intends to use them in combat or to support combat operations against Ukrainian forces in the Russian Kursk region near the border with Ukraine," where Russia is attempting to repel a Ukrainian incursion.

For his part, U.S. President Joe Biden on Monday considered the sending of thousands of soldiers by North Korea to Russia a "very serious development," in comments to reporters after casting his early vote in the U.S. presidential elections in his home state of Delaware.

Earlier on Monday, NATO Secretary-General Mark Rutte confirmed recent Ukrainian intelligence reports indicating that some North Korean military units were already in the Kursk region.

Days earlier, the head of Ukrainian intelligence stated that there are nearly 11,000 North Korean soldiers training in eastern Russia to fight in Ukraine.

South Korean intelligence reported that the northern neighbor has decided to send "a large force of soldiers" to Russia to support it in the war, and that 1,500 North Korean soldiers are training in eastern Russia.

Western officials say that sending thousands of North Korean soldiers to Ukraine threatens to increase pressure on the exhausted Ukrainian army and escalate geopolitical tensions in the Korean Peninsula and the Indo-Pacific region, including Japan and Australia.

An analysis from "Voice of America" stated that U.S. officials are increasingly concerned about the emerging partnership between China, Russia, Iran, and North Korea, which some in Washington describe as the "new axis of evil."