Pentagon spokesman John Kirby said Russia had not announced, as is traditional, that it was conducting exercises, leaving its intentions ambiguous and firing up tensions in the region.

"We call on Russia to make their intentions more clear as to what they're doing with this array of forces along the border," Kirby told reporters in a briefing.

"What's important is... to de-escalate the tensions along that border," said Kirby.

All sides need to abide by the 2014-2015 Minsk agreements on the Ukraine conflict with Russia-aligned rebels, "and to bring the temperature down and to de-escalate," he added.

Kirby would not give any details of what the United States knows of the deployments. Unconfirmed videos on social media show significant amounts of Russian military vehicles and armor driving or being freighted into the area.

Kirby said the biggest US concern is the increased Russian presence on Ukraine's southeast border and in Crimea, the Ukraine territory that Moscow seized in 2014.

"We continue to see Russian forces arrayed along the border with Ukraine, in Crimea, specifically, so more towards the southeast, and we're monitoring that very, very closely," he said.

On Monday State Department Spokesman Ned Price said Washington remained very concerned about "escalating Russian aggressions in eastern Ukraine."

"At the highest levels of government, literally, across multiple institutions, we have sent that message very clearly to our Ukrainian counterparts, and implicitly to the Russians as well, that we stand by Kiev, we stand by our partner Ukraine, in the face of this intimidation and aggression," Price said.