The United States warned on Tuesday of "serious consequences" for any renewed Russian aggression as the Western military alliance NATO met to discuss Moscow's intentions for massing troops on the border with Ukraine.

U.S. Secretary of State Antony Blinken was due to brief his 29 NATO counterparts on the U.S. intelligence picture on the group's eastern flank and in Ukraine, which is not a member.

At a news conference in Latvia before the meeting, Blinken expressed alarm at the "unusual" Russian troop movements.