Russia's claim that it is moving troops away from the border with Ukraine is "false", according to a senior US official, who added that 7,000 extra troops have arrived in recent days.
The official also said that Russia could launch a "false" pretext to invade Ukraine "at any moment".
Moscow says it is moving troops away from the Ukrainian border after the completion of military exercises.
But Western officials say they have seen no evidence to support the claim.
"Russia must take real steps toward de-escalation," US President Joe Biden and German Chancellor Olaf Scholz agreed in a phone call on Wednesday, according to the German chancellery.
Russia - which has repeatedly denied planning to invade Ukraine, despite having amassed well over 100,000 troops near the border - calls Western concern over an invasion "hysteria".
Its defence ministry has published images purportedly showing troops and equipment returning to their permanent bases after military exercises.
But according to a senior White House official, thousands more troops have arrived in the area in recent days - including some on Wednesday itself.
Speaking to journalists, the US official cast doubt on Russia's assertions that "it was withdrawing troops from the border with Ukraine".
"They received a lot of attention for that claim, both here and around the world. But we now know it was false."
It came hours after Ukrainian President Volodymyr Zelensky told the BBC: "We don't see any troop withdrawal yet, we just heard about it."
He was speaking as Ukraine marked a so-called day of unity on Wednesday, with national blue and yellow flags raised throughout the country.
President Zelensky declared the patriotic holiday in response to US intelligence reports that Russia could attack Ukraine on the same day.
Nato Secretary-General Jens Stoltenberg has also said there are no signs that Russian forces are de-escalating, saying on Wednesday that the threat from Russia had become a "new normal".
Speaking at a summit of Nato defence ministers in Brussels, Mr Stoltenberg said the alliance was considering setting up new battle groups - the smallest type of self-sufficient military units - in central and south-eastern Europe.
He said this was part of ongoing measures to bolster European defence - on which $270bn (£199bn) had been spent since 2014 - although he attempted to reassure Russia that Nato was not a threat.
France had offered to lead one such battle group in Romania, he said.
Russia's foreign ministry said it was "no longer interested" in Mr Stoltenberg's statements.
The scepticism over Russia's claimed withdrawal was also echoed in the UK. Writing in the UK newspaper The Telegraph on Thursday, Foreign Secretary Liz Truss - who is expected to travel to Ukraine shortly - warned the West should not be lulled into a false sense of security.
Prime Minister Boris Johnson earlier told the UN Secretary-General in a phone call there was "currently little evidence of Russia disengaging", while his defence intelligence chief Lt-Gen Jim Hockenhull said that, on the contrary, Russia was continuing to build up its forces.
"This includes sightings of additional armoured vehicles, helicopters and a field hospital moving towards Ukraine's borders," he said. "Russia has the military mass in place to conduct an invasion of Ukraine."
On Wednesday, the US State Department warned that Russian officials were planting stories in the media aimed at influencing public opinion in the event of an invasion.
Spokesman Ned Price expressed concern at Mr Putin's claim that "genocide" was taking place in eastern Ukraine, where Russian-backed separatists and Ukrainian forces have been in conflict since 2014.
Russian officials said they were investigating alleged mass graves containing hundreds of civilians in the conflict zone.
Early on Thursday, Russian media reported that Ukrainian forces had violated a ceasefire agreement by firing mortars at rebel forces in the Luhansk district. There has been no independent verification of the claim and Ukraine has denied shelling the area.
Russia and Ukraine have deep cultural and historic ties, and both were part of the former Soviet Union.
Russian President Vladimir Putin wants assurances that Ukraine will not join Nato because he sees any expansion of it as a threat. Nato has rejected that demand.
Earlier this week, Mr Putin said that Russia did not want war, but demanded that the issue of Nato membership be addressed now, even though Ukraine is a long way from even starting an application to join the alliance.
BDST: 1254 HRS, FEB 17, 2022