At least 3,000 North Korean troops have been sent to training locations in eastern Russia, according to the United States.
The troops were transported by ship from Wonsan, North Korea to Vladivostok earlier this month.
"We do not yet know whether these soldiers will enter into combat alongside the Russian military, but this is certainly a highly concerning probability," said National Security Council spokesperson John Kirby.
Ukrainian officials released footage on Tuesday reportedly showing North Korean soldiers carrying Russian military uniforms.
According to Ukrainian military intelligence head Kyrylo Budanov, 11,000 North Korean soldiers could be training to aid Russia in its invasion.
Ukrainian intelligence also recently reported that North Korean officers were already working with the Russian military in Ukraine.
South Korea’s National Intelligence Service concurred, saying on Friday they had identified dozens of North Korean officers assisting Russian soldiers with missile operation on the Ukrainian frontlines.
“We believe that the possibility of further deployment of regular troops is very high, as Russia and North Korea have entered a mutual agreement that is almost equivalent to a military alliance,” said South Korean Defence Minister Kim Yong-hyun after reports of North Korean casualties in Ukraine.
North Korea and Russia signed a mutual defense pact in June.
According to South Korean intelligence, North Korea has sent 70 shipments of munitions to Russia since last August, including 8 million rounds of arms.