White House national security spokesperson John Kirby says at least 3,000 North Korean soldiers are now being trained in facilities in eastern Russia, possibly to fight for Russia.

Kirby told reporters on Wednesday, "We assess that between early to mid-October, North Korea moved at least 3,000 soldiers into eastern Russia."

He said the US assesses the soldiers traveled by ship from the Wonsan area in eastern North Korea to Vladivostok in Russia's Far East, and then to multiple military training sites in eastern Russia, where they are currently undergoing training.

He added that it is "certainly a highly concerning probability" that these soldiers could later be sent to western Russia and engage in combat against the Ukrainian military.

Kirby pointed out that this would show Russia is suffering extraordinary casualties on the battlefield every day.

He said it would also demonstrate "an unprecedented level of direct military cooperation between Russia and North Korea, with security implications in Europe, as well as the Indo-Pacific."

Kirby said the US, together with its allies and partners, will keep a close eye on the movement of North Korean soldiers being trained in Russia.

US Defense Secretary Lloyd Austin has also expressed grave concern over North Korea's troop dispatch to Russia, calling it "very, very serious."