The United States has renewed concerns about Russian military maneuvers along its border with Ukraine, charging that Moscow has now massed more troops in the area than when it invaded and seized Crimea seven years ago.

"It is certainly bigger than that,” Pentagon press secretary John Kirby told reporters Monday, calling the ongoing buildup of Russian forces "very seriously concerning," Voice of America reports.

"In general, we have continued to see this buildup increase," Kirby said. "We certainly heard the Russians proclaim that this is all about training. It's not completely clear to us that that's exactly the purpose."

Earlier, Pentagon assessed the Russian troop buildup as merely the largest since the 2014 invasion of Crimea. The new one follows similar intelligence assessments by European Union officials, who alleged there are now more than 150,000 Russian troops in the area.

"It is the highest military deployment of the Russian army in Ukrainian borders ever," EU foreign policy chief Josep Borrell said earlier Monday, adding that the risk of further escalation is "evident" and accusing Russia of "deploying campaign hospitals and all kinds of warfare" to the region.

Last week, the top U.S. military commander for Europe said it appeared Russian deployment "has plateaued," calling the risk of a Russian incursion over the next week and a half as "low to medium."

The U.S. and its Western allies, however, continue to call on Russia to "cease their provocations" and de-escalate.

They have also been critical of Russian plans to limit access later this month to the Black Sea and the Kerch Strait, with the Kremlin citing ongoing military exercises.