Welcoming Ukraine's foreign minister to Washington, Secretary of State Antony Blinken said that the United States was "concerned with reports of the unusual Russian activity near Ukraine."

"We don't have clarity into Moscow's intentions, but we do know its playbook," Blinken told a joint news conference.

"Our concern is that Russia may make the serious mistake of attempting to rehash what it undertook back in 2014 when it amassed forces along the border, crossed into sovereign Ukrainian territory and did so claiming falsely that it was provoked," he said.

"Our commitment to Ukraine's sovereignty, to its independence, to its territorial integrity is ironclad, and the international community will see through any Russian effort to resort to its previous tactics," Blinken said.

Ukraine has been ensnared in a deadly war with pro-Moscow separatists in its largely Russian-speaking east since 2014 when Russia seized the Crimea peninsula.

Russia took action after mass protests led to the ousting of the president who had suspended efforts to move closer to the European Union.

Russia in March also amassed 100,000 troops on Ukraine's borders. It pulled back, but both Ukraine and the United States said at the time that the withdrawal was limited.

- Seeking clarity -

Foreign Minister Dmytro Kuleba said Ukraine was looking to work with the United States to step up its defenses and "does not intend to attack anyone."

"The best way to deter an aggressive Russia is to make it clear for the Kremlin that Ukraine is strong, but also that it has strong allies that will not leave it on its own in the face of Moscow's ever increasing aggressiveness," Kuleba said.

"Russian aggression against Ukraine will end on the day Ukraine's place as part of the West is institutionalized and undoubted," he said.

Pentagon spokesman John Kirby said that the movement by Russia was "unusual in its size and scope."

"We urge Russia to be clear about their intentions and to abide by their Minsk agreements," he said, referring to agreements to try to halt the fighting inside Ukraine.

The comments followed a rare visit to Moscow last week by the director of the US Central Intelligence Agency, William Burns, who met with President Vladimir Putin.

CNN reported that President Joe Biden deployed Burns, the former US ambassador to Moscow, to raise the issue of the Russian troop surge directly with the Kremlin.