WASHINGTON - The United States 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."

"In general, we have continued to see this buildup increase," he 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."

The Pentagon's assertion is a slight change from previous statements, when it categorized the Russian troop buildup as merely the largest since the 2014 invasion of Crimea. And it 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.

"The risk of further escalation is evident," Borrell added, accusing Russia of "deploying campaign hospitals and all kinds of warfare" to the region.

Last week, Air Force General Tod Wolters, the top U.S. military commander for Europe, told U.S. lawmakers it appeared activity along the Russian supply lines "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.

In response, the U.S. called Moscow's move "just the latest example" of the Kremlin's aggression and said it would continue to operate warships in the Black Sea as it saw fit.

The Sunday Times, citing British naval sources, reported that Britain plans to send two vessels to the Black Sea sometime next month.