The US Department of Defense is investigating reports of recent Russian troop movements near the border with Ukraine.

The Pentagon is following developments in the region closely, said its spokesman John Kirby on Monday.

According to the Washington Post, US officials are concerned about movements by Russian military units towards the border with Ukraine.

Videos distributed on the Internet show military trains and truck convoys loaded with tanks and missiles in southwest Russia.

"We know of public reports of unusual Russian military activities near Ukraine," said Kirby.

Washington is in contact with its allies and partners on the issue.

In March, Russia gathered thousands of soldiers, heavy military equipment, naval ships and air force planes near the Ukrainian border and on the Russian-annexed Crimean peninsula in a massive deployment of troops.

The massive troop movements fueled fears that there could be a Russian invasion of Ukraine.

After weeks, Moscow finally declared the alleged maneuver over and withdrew its soldiers.

The Ukrainian army has been fighting pro-Russian separatists in the east of the country since 2014.

The West accuses Russia of supporting the separatists, which the government in Moscow denies.