The United States on Wednesday dismissed reports that Russia was withdrawing troops from Ukraine's border, instead accusing Moscow of sending more soldiers as fears of an invasion grow.

Russia has increased its presence on the border with Ukraine by "as many as 7,000 troops," some of whom arrived Wednesday, said a senior White House official, slamming Moscow's announcement of a withdrawal as "false."

"We continue to receive indications they could launch a false pretext at any moment to justify an invasion."

The US embassy moved its diplomatic activities from Kyiv to Lviv in western Ukraine
The US embassy moved its diplomatic activities from Kyiv to Lviv in western Ukraine AFP / Aris Messinis

The official, who requested anonymity, added that while Moscow has said it wants to reach a diplomatic solution, its actions "indicate otherwise."

Earlier Wednesday, the United States and NATO joined Ukraine in saying there was no sign of Russian troops withdrawing after military movements in occupied Crimea fueled reports that the crisis could be abating.

Ukraine's President Volodymyr Zelensky marked what he had declared "Day of Unity" by watching Ukrainian soldiers train with new Western-supplied anti-tank weapons near Rivne, west of the capital Kyiv.

The Russian defence ministry released images of tanks departing by train after concluding military exercises
The Russian defence ministry released images of tanks departing by train after concluding military exercises Russian Defence Ministry via AFP / Handout

He also visited the frontline city of Mariupol, wearing a military-style olive green coat.

"We are not afraid of anyone, of any enemies," Zelensky said on a day that Western intelligence had warned Moscow could choose to invade. "We will defend ourselves."

Despite images on Russian state media that were said to show Moscow's forces winding up a major exercise in Crimea, Zelensky said there was no evidence of Russians pulling back.

Russia bolsters military deployment on the borders with Ukraine
Russia bolsters military deployment on the borders with Ukraine AFP / Patricio ARANA

"We are seeing small rotations. I would not call these rotations the withdrawal of forces by Russia," he said in televised comments. "We see no change."

In Rivne, missiles pounded practice targets, while in Kyiv hundreds of civilians marched in a stadium with an enormous national banner.

Russia's huge build-up of troops, missiles and warships around Ukraine has been billed as Europe's worst security risk since the Cold War.

Washington has warned that any invasion of Ukraine would have severe consequences for Russia
Washington has warned that any invasion of Ukraine would have severe consequences for Russia AFP / Brendan Smialowski

NATO chief Jens Stoltenberg, who hosted a meeting of alliance defense ministers in Brussels, also dismissed suggestions that the threat on Ukraine's border had diminished.

"Moscow has made it clear that it is prepared to contest the fundamental principles that have underpinned our security for decades and to do so by using force," he said.

"I regret to say that this is the new normal in Europe."

NATO says claims of a Russian pullback remain to be confirmed
NATO says claims of a Russian pullback remain to be confirmed AFP / Kenzo TRIBOUILLARD

On the reported Russian troop movements, he said: "So far we do not see any sign of de-escalation on the ground.

In Rivne, missiles pounded targets and armoured vehicles manoeuvred and fired on the yellowing moorland
In Rivne, missiles pounded targets and armoured vehicles manoeuvred and fired on the yellowing moorland Ukrainian Defence ministry press-service via AFP / Handout

"Russia maintains a massive invasion force ready to attack with high-end capabilities from Crimea to Belarus."

German Chancellor Olaf Scholz echoed Wednesday that "the risk of a further military aggression by Russia" remains "high," according to a statement issued following a phone call with US President Joe Biden.

Russian President Vladimir Putin has demanded Ukraine be forbidden from pursuing its ambition to join NATO and wants to redraw the security map of eastern Europe, rolling back Western influence.

But, backed by a threat of crippling US and EU economic sanctions, Western leaders are pushing for a negotiated settlement, and Moscow has signaled it will start to pull forces back.

In the latest such move, the Russian defense ministry said on Wednesday that military drills in Crimea -- a Ukrainian region that Moscow annexed in 2014 -- had ended and that troops were returning to their garrisons.

While Washington has demanded verifiable evidence of de-escalation, Biden has nevertheless vowed to push for a diplomatic solution.

Kremlin spokesman Dmitry Peskov welcomed this, telling reporters: "It is positive that the US president is also noting his readiness to start serious negotiations."

Meanwhile, the Pentagon said that three US Navy aircraft were intercepted by Russian planes in an "unprofessional" manner over the Mediterranean Sea last weekend.

The US State Department had said earlier that Russia was attempting to create a pretext for invading with unsupported claims of "genocide" and mass graves in Ukraine's eastern Donbas region, which is controlled by Moscow-backed separatists.

"Over the past several weeks, we've also seen Russian officials and Russian media plant numerous stories in the press, any one of which could be elevated to serve as a pretext for an invasion," State Department Spokesman Ned Price said.

EU leaders, already gathered in Brussels for a summit with their African counterparts, are now to hold impromptu crisis talks Thursday on Russia and Ukraine.

A UN Security Council meeting is also set Thursday to discuss the crisis.

And US Vice President Kamala Harris will meet with Zelensky on the sidelines of the annual Munich Security Conference this weekend, a senior White House official said Wednesday.

"It cannot be excluded that the aggressor is resorting to dirty tricks," Ukraine's communications watchdog said, referring to Russia.

Kremlin spokesman Peskov denied that Moscow had any role in the cyber assault and accused Ukraine of "blaming Russia for everything."