"White House national security spokesman John Kirby said the U.S. believes that at least 3,000 North Korean soldiers traveled by ship to Vladivostok, Russia's largest Pacific port, in early to mid-October.

"These soldiers then traveled onward to multiple Russian military training sites in eastern Russia, where they are currently undergoing training," Kirby said.

"We do not yet know whether these soldiers will enter into combat alongside the Russian military, but this is certainly a highly concerning probability."