New commercial satellite images show a flurry of Russian military activity at several locations near Ukraine, the private U.S. company that released the pictures said, amid fears that Moscow may launch an attack on its ex-Soviet neighbour.

Russia, which has repeatedly denied plans to invade, is holding large military exercises in ex-Soviet Belarus, part of a dramatic buildup of forces to the north, east and south of Ukraine.

U.S.-based Maxar Technologies, which has been tracking the buildup of Russian forces for weeks, said on Tuesday that images taken on Sunday and Monday captured significant new activity in Belarus, annexed Crimea and western Russia.

The images could not be independently verified by Reuters.

Maxar pointed to the arrival of several large deployments of troops and attack helicopters as well new deployments of ground attack aircraft and fighter-bomber jets to forward locations.

The images also captured the departure of multiple ground forces units from existing garrisons along with other combat units seen in convoy formation, Maxar said.