Russia amasses troops near U.S. ally Ukraine. But what is Putin's goal?

Tens of thousands of Russian troops massing near the Ukrainian border, convoys of tanks, and a deadly escalation in the grinding trench war in eastern Ukraine.

These storm clouds on Europe's eastern flank are causing grave alarm in Washington and across the continent.

"We're now seeing the largest concentration of Russian forces on Ukraine's borders since 2014," Secretary of State Antony Blinken said Tuesday after flying to NATO's headquarters in Brussels, Belgium. "That is a deep concern not only to Ukraine, but to the United States."

In a call with Russian President Vladimir Putin later in the day, President Joe Biden declared Washington's "unwavering commitment to Ukraine's sovereignty and territorial integrity" and "called on Russia to de-escalate tensions," a White House readout said.