The U.S., South Korean and Japanese national security advisers expressed "grave concern" on Friday over the deployment of North Korean troops in Russia for possible use against Ukraine, the White House said.

The trio see the North Korean troop presence as the latest sign of growing Moscow-Pyongyang military ties, and they called on Russia and North Korea to cease arms and missile transfers, White House national security spokesman John Kirby told reporters.

The transfers violate U.N. Security Council resolutions, he said.