These 3,000 troops from the 82nd Airborne Division, the US military's main rapid reaction force, were already placed on alert in late January at the request of President Joe Biden.

Biden wanted to show support for US allies in Eastern Europe in the face of any possible aggression by Russia.

They will now leave the base in coming days and are expected to arrive in Poland early next week, a senior defense official said.

These troops will join some 2,000 airborne soldiers whose deployment was announced February 2. A total of 1,700 are in Poland and the rest in Germany, where US forces in Europe have their headquarters.

"All told, these 5,000 additional personnel comprise a highly mobile and flexible force, capable of multiple missions," said the US official, who spoke on condition of anonymity.

"They are being deployed to reassure our NATO allies, deter any potential aggression against NATO's eastern flank, train with host-nation forces, and contribute to a wide range of contingencies," the official added.

They are in addition to some 80,000 US troops stationed in Europe, either on a permanent base or on a rotating basis.

In a dramatic escalation of its threat warning, the White House said Friday that Russia could invade Ukraine in a matter of days, even before the end of the Beijing Olympics on February 20.