About 10,000 North Korean soldiers were believed to be based in the Russian border region of Kursk, Austin said, where they were being "integrated into the Russian formations".

"Based upon what they've been trained on, the way they've been integrated into the Russian formations, I fully expect to see them engaged in combat soon," Austin told reporters during a stopover in Pacific nation Fiji.

Austin said he had "not seen significant reporting" of North Korean troops being "actively engaged in combat" to date.

South Korean government officials and a research group on Thursday said Russia has provided Pyongyang with oil, anti-air missiles and economic help in exchange for the troops Washington and Seoul have accused it of sending.

Kyiv has warned that Moscow, alongside the North Korean soldiers, has now amassed a 50,000-strong force to wrest back parts of the border region seized by Ukrainian forces.

Ukraine claimed swathes of Kursk in August during a lightning offensive even as its troops were thinly stretched in the Donetsk region, which has borne the brunt of nearly three years of fighting