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

Earlier on Wednesday, the United States and Nato joined Ukraine in saying there was no sign of Russian troops withdrawing after military movements in occupied Crimea fuelled 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.

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.

“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.”

Read more: Russia says pulling back some forces from Ukraine border

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.

Nato chief Jens Stoltenberg, who hosted a meeting of alliance defence 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.”

'Invasion force ready'

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

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

German Chancellor Olaf Scholz echoed on 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 signalled it will start to pull forces back.

In the latest such move, the Russian defence 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.”

US slams invasion 'pretext'

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

Also read: Ukraine-Russia crisis: What to know as diplomacy steps up

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 for 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 on Wednesday.

Ukraine had said on Tuesday the websites of the country's defence ministry and armed forces, as well as private banks, had been hit by a cyberattack of the kind that US intelligence fears would precede a Russian attack.

“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”.