Russia could invade Ukraine at any time using a fabricated pretext or a false flag operation, the White House warned on Wednesday

"We're in the window where we believe an attack could come at any time and that would be preceded by a fabricated pretext that the Russians use as an excuse to launch an invasion," White House press secretary Jen Psaki said at a news conference. "And we've seen and we've talked about this a bit in here. We've seen these tactics used in the past."

Her remarks came one day after Russian defense officials reported that some military units were leaving their positions near Ukraine's border. But U.S. Secretary of State Antony Blinken said Wednesday that the U.S. does not see any "meaningful pullback" of Russian troops.

Moscow, according to Ukrainian officials and NATO, has recently amassed more than 100,000 troops near Ukraine, prompting fears that the Kremlin could be planning a military offensive against its ex-Soviet neighbor.

Denying that it is preparing to invade, Russia has accused Western countries of undermining its security through NATO's expansion toward its borders.

Russia also issued a list of security demands to the West, including a rollback of troop deployments from some ex-Soviet states, and guarantees that some of those states would not join NATO.

In a written response to the demands, Washington said it is committed to upholding NATO's "open door policy," while NATO also conveyed the alliance's reply "in parallel with the United States."