Pentagon spokesman John Kirby said the US was concerned about troop movements inside Russia's borders but close to the frontier of Ukraine, where Ukraine government forces are locked in a seven-year war with pro-Moscow rebels.

Kirby did not provide details about what the US military had observed of Russian military activities, saying only that it was "unusual in its size and scope."

"It's not exactly clear what the Russian intentions are," he said.

Washington was keen to "better understand" the activities and did not want to see "any action further destabilize what is already a very tense part of the world," Kirby added.

"We urge Russia to be clear about their intentions and to abide by their Minsk agreements,' he said, referring to agreements to try and halt the fighting inside Ukraine.

The United States is keeping a close watch on the developments.

"Our support for the territorial integrity and sovereignty of Ukraine is unwavering," Kirby added.

The comments followed a rare visit to Moscow last week by the director of the US Central Intelligence Agency, William Burns, who met in the Russian capital with President Vladimir Putin.

CNN reported that President Joe Biden deployed Burns, the former US ambassador to Moscow, to raise the issue of the Russian troop surge directly.