A U.S. government official on Wednesday called Russia's claim of a partial troop withdrawal near Ukraine "false," shrugging off any optimism about a de-escalation of the massive Russian military buildup near Ukraine that has raised fears of an invasion.

In the last several days, Russia has increased its military presence by as many as 7,000 troops, the official said, adding that some even arrived on Wednesday.

U.S. President Joe Biden speaks at the White House in Washington on Feb. 15, 2022. (UPI/Kyodo)

The official also said Russia's recently expressed readiness to engage in diplomacy can be seen as a pretense.

"Every indication we have now is they mean only to publicly offer to talk, and make claims about de-escalation, while privately mobilizing for war," the official said.

Russia announced Tuesday that some troops were returning to their bases following exercises near Ukraine. Its Defense Ministry also said Wednesday that troops were leaving Crimea after drills.

But the United States has said more than 150,000 troops are arrayed along Ukraine's borders -- inside Russian territory close to the Ukrainian border and also inside Belarus, which has close ties with Moscow.

"More Russian forces, not fewer, are at the border. And they're moving, concerningly, into fighting positions," State Department spokesman Ned Price said at a press conference.

NATO Secretary General Jens Stoltenberg told a separate press conference after the first day of its defense ministerial meeting in Brussels that countries "do not see any sign of de-escalation on the ground" and there have been "no withdrawals of troops or equipment."

"This is the biggest concentration of forces in Europe since the Cold War," the head of the North Atlantic Treaty Organization added.

While Wednesday passed without incident despite reports by some U.S. and European media outlets on speculation that an invasion could begin that day, Price maintained the U.S. stance that Russian military action could be imminent.

"It could take place tomorrow, it could take place next week, it could take place before the end of the Olympics, it could take place after the Olympics," he said, referring to the global sporting event being held in Beijing through Sunday.

Russia has been asserting that its security is under threat with NATO's eastward expansion and the possibility of Ukraine's membership in the alliance, although it has denied its intention to invade the former Soviet republic.

The United States has demanded Russia de-escalate by removing its troops from the border, while rejecting Moscow's key demand to offer a security guarantee precluding Ukraine's entry into NATO.

But at the same time, Washington has suggested room for discussions with Russia to address their security concerns such as by setting reciprocal limits on military exercises in Europe and missile placement.

The United States and its allies have been calling for a diplomatic solution to the crisis, warning that any further Russian aggression against Ukraine, following its annexation of Crimea in 2014, would be met with severe costs including heavy economic sanctions from the West.

Japan, a close U.S. ally, told the United States it is committed to closely coordinating with the United States and the Group of Seven industrialized countries to respond to further Russian aggression against Ukraine, the White House said in a press release issued after a phone call between the two countries' national security advisers.

U.S. National Security Adviser Jake Sullivan told his Japanese counterpart Takeo Akiba that Russian forces remain postured to "attack Ukraine at any time," and underscored the importance of a "strong international response" to any Russian incursion, it said.