President Biden has approved the deployment of about 3,000 additional American troops to Eastern Europe, administration officials said on Wednesday.

The troops, including 1,000 already in Germany, will head to Poland and Romania, said the Pentagon spokesman, John F. Kirby. Their purpose will be to reassure NATO allies that while the United States has no intention of sending troops into Ukraine, where President Vladimir V. Putin of Russia has been threatening an invasion, Mr. Biden would protect Americaxe2x80x99s NATO allies from any Russian aggression.

xe2x80x9cIts important that we send a strong signal to Mr. Putin and the world that NATO matters,xe2x80x9d Mr. Kirby told reporters at a news conference. xe2x80x9cWe are making it clear that we are going to be prepared to defend our NATO allies if it comes to that.xe2x80x9d

At the moment, Russia is threatening Ukraine, not Romania or Poland. But Mr. Putin has made clear his distaste for both NATO and the post-Cold War redrawing of the map of Europe, which put former Soviet republics and satellite countries in the Westxe2x80x99s foremost military alliance at his doorstep.

The presidentxe2x80x99s decision comes days after Pentagon leaders said that Mr. Putin had deployed the necessary troops and military hardware to conduct an invasion of Ukraine. Senior Defense Department officials also said that the tense standoff was leading the United States, its NATO allies and Russia into uncharted territory.

The number of Russian troops assembled at Ukrainexe2x80x99s borders has reached well north of 100,000, the officials said, publicly confirming for the first time what intelligence analysts have described for weeks.

Close to 2,000 of the troops xe2x80x94 most of them coming from the 82nd Airborne in Fort Bragg, N.C. xe2x80x94 will be going to Poland, Mr. Kirby said. While many of those troops are paratroopers, Mr. Kirby said he did not expect the Airborne troops to deploy to Poland in a xe2x80x9ctactical operation,xe2x80x9d which would raise the ire of Russia even more.

The troops being moved to Romania will complement French troops being deployed there, Mr. Kirby said.

The Biden administration has not ruled out sending additional troops to Europe, and still has 8,500 American troops on xe2x80x9chigh alertxe2x80x9d for possible deployment to a NATO rapid response force.

Asked why Mr. Biden decided to move troops unilaterally, Jen Psaki, White House press secretary, said the move had been under discussion xe2x80x9cfor some timexe2x80x9d with NATO partners and there was no specific event in recent days that pushed Mr. Biden to deploy the troops.

xe2x80x9cTherexe2x80x99s no question that Russia and President Putin has continued to take escalatory, not de-escalatory steps,xe2x80x9d Ms. Psaki said. xe2x80x9cSo it is not that it is one moment, it is we are looking at events over the course of time.xe2x80x9d

Mr. Kirby also said there would be no change in the status of the small number of American troops in Ukraine. More than 150 U.S. military advisers are in Ukraine, trainers who have for years worked near Lviv, in the countryxe2x80x99s west, far from the front lines. The current group includes Special Operations forces, mostly Army Green Berets, as well as National Guard trainers from Floridaxe2x80x99s 53rd Infantry Brigade Combat Team.

xe2x80x9cItxe2x80x99s a big, unambiguous signal,xe2x80x9d said Jim Townsend, a former top Pentagon official for Europe and NATO policy. xe2x80x9cItxe2x80x99s also significant that they are going to the Black Sea. Finally, the Black Sea region is being recognized as a major theater. Itxe2x80x99s not just the Baltics.xe2x80x9d

Zolan Kanno-Youngs contributed reporting.