According to The Washington Post, US officials are concerned over unexplained movements of Russian military units in the area.

Videos posted on social media show military trains and truck convoys moving tanks and missiles in Russia's southwest near Ukraine.

A US official, speaking on the basis of anonymity, confirmed that there had been some movement, though not as large as the surge in Moscow's forces early this year.

"We're aware of public reports of unusual Russian military activity near Ukraine," Kirby told reporters.

"We're going to continue to consult with allies and partners on this issue... As we've said before, any escalatory or aggressive actions would be of great concern to the United States."

The buildup comes just after US Defense Secretary Lloyd Austin toured the Black Sea region, stopping in NATO partner states Ukraine and Georgia and alliance full member Romania, to express solidarity and build more support for confronting Russia in the area.

Russia-aligned separatists in eastern Ukraine have been battling Ukrainian government forces since 2014.

Also in 2014, Russian troops seized the Crimea region of Ukraine, sparking stiff Western sanctions on the country.

In March, just weeks after President Joe Biden's inauguration, Russia massed thousands of troops, heavy military equipment, naval vessels and aircraft on the Ukraine border and in Crimea, holding them there for weeks and sparking fears of an invasion.

Moscow called the movements an exercise, but many of the military assets remained for weeks, and analysts saw it as a warning to Biden.

Washington has refused to accept Russia's claim on Crimea and has delivered lethal military aid to Kiev for use in fighting the separatists.

Moscow has made clear its rejection of any effort to bring Ukraine, a former part of the Soviet Union, into the NATO alliance.

Kirby said that Washington would consult allies and partners on the reported Russian buildup.

"We continue to support deescalation in the region and a diplomatic resolution to the conflict in eastern Ukraine," he said.