The pictures show "multiple new field deployments of armored equipment and troops" departing from existing military sites into forests and fields approximately 9 to 19 miles from Russia's border with Ukraine, US satellite imagery company Maxar said in an email.

The new photos, taken Sunday, appear to show vehicle tracks cutting through snow-covered fields that are surrounded by woodland and bordered by roads, with several buildings also visible.

Analysis of Sunday's photos, plus others taken February 13, indicate troop and equipment movement near three localities in southwestern Russia, according to the Colorado-based satellite company.

"Today, most of the combat units and support equipment at Soloti have departed and extensive vehicle tracks and some convoys of armored equipment are seen throughout the area," Maxar said.

"Some equipment has also been deployed east of nearby Valuyki, Russia in a field approximately 15 kilometers north of the Ukraine border."

"Moreover, a number of new field deployments are also seen northwest of Belgorod (deployments are approximately 30 kilometers from the border with Ukraine) with much of the equipment and troops positioned in or near forested areas," the US firm said. "Other company-sized units are deployed within farm and/or industrial areas."

Russia has placed more than 150,000 troops near Ukraine's borders in recent weeks, the United States and Western allies have estimated, with Washington warning an invasion is imminent.

"Forty to fifty percent (of those troops) are in an attack position. They have uncoiled in tactical assembly in the last 48 hours," a US official told reporters Friday, insisting on anonymity.

Moscow denies it has plans to attack its western neighbor, but is seeking a guarantee that Ukraine will never join NATO and that the Western alliance remove forces from Eastern Europe, demands the West has refused.