The Pentagon is sending another 3,000 combat troops to Poland to join 1,700 who already are assembling there in a demonstration of American commitment to NATO allies worried at the prospect of Russia invading Ukraine, a senior defense official said Friday.

The official, who provided the information on condition of anonymity before an official announcement, said the additional soldiers will depart their post at Fort Bragg, North Carolina, over the next couple of days and should be in Poland by early next week. They are the remaining elements of an infantry brigade of the 82nd Airborne Division.

The announcement came shortly after Jake Sullivan, President Joe Biden's national security adviser, issued a public warning for all American citizens in Ukraine to leave the country as soon as possible. Sullivan said Russian President Vladimir Putin could give the order to launch an invasion of Ukraine any day now.

In addition to the U.S. troops deploying to Poland, about 1,000 U.S. soldiers based in Germany are shifting to Romania in a similar mission of reassurance to a NATO ally.