"We continue to watch the situation and the information we gathered so far is rather worrying," EU foreign affairs spokesman Peter Stano told journalists.

He said the 27-nation bloc was monitoring the situation with partners including the United States and Britain and "we are open to look at further steps as necessary".

US top diplomat Antony Blinken on Wednesday warned Russia against making another "serious mistake" on Ukraine as Washington sought clarity about troop movements near the border.

Welcoming Ukraine's foreign minister to Washington, Secretary of State Blinken said that the US 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.

Pentagon spokesman John Kirby said the Russian movements were "unusual in its size and scope."

"It's not exactly clear what the Russian intentions are," he said.

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

EU chief Ursula von der Leyen discussed the situation around Ukraine with US President Joe Biden during a visit to Washington this week.

"The EU and the US fully support the territorial integrity of Ukraine," von der Leyen tweeted after the meeting.

"And we are fully behind them in their efforts to modernise their economy and build up resilience."

An official at US-led NATO said the alliance "is vigilant and routinely monitors Russian force movements. It's important to ensure transparency and avoid any miscalculation."

Ukraine's foreign minister is set to hold talks with NATO head Jens Stoltenberg in Brussels on Monday.