WASHINGTON — President Biden is considering deploying several thousand U.S. troops, as well as warships and aircraft, to NATO allies in the Baltics and Eastern Europe, an expansion of American military involvement amid mounting fears of a Russian incursion into Ukraine, according to administration officials.

The move would signal a major pivot for the Biden administration, which up until recently was taking a restrained stance on Ukraine, out of fear of provoking Russia into invading. But as President Vladimir V. Putin has ramped up his threatening actions toward Ukraine, and talks between American and Russian officials have failed to discourage him, the administration is now moving away from its do-not-provoke strategy.

In a meeting on Saturday at Camp David, the presidential retreat in Maryland, senior Pentagon officials presented Mr. Biden with several options that would shift American military assets much closer to Mr. Putin’s doorstep, the administration officials said. The options include sending 1,000 to 5,000 troops to Eastern European countries, with the potential to increase that number tenfold if things deteriorate.

The officials spoke on the condition of anonymity because they were not authorized to talk publicly about internal deliberations.

Mr. Biden is expected to make a decision as early as this week, they said. He is weighing the buildup as Russia has escalated its menacing posture against Ukraine, including massing more than 100,000 troops and weaponry on the border and stationing Russian forces in Belarus. On Saturday, Britain accused Moscow of developing plans to install a pro-Russian leader in Ukraine.

“Even as we’re engaged in diplomacy, we are very much focused on building up defense, building up deterrence,” Secretary of State Antony J. Blinken said in an interview that aired Sunday on CBS’s “Face the Nation.” “NATO itself will continue to be reinforced in a significant way if Russia commits renewed acts of aggression. All of that is on the table.”

So far, none of the military options being considered include deploying additional American troops to Ukraine itself, and Mr. Biden has made clear that he is loath to enter another conflict following America’s painful exit from Afghanistan last summer after 20 years.

But after years of tiptoeing around the question of how much military support to provide to Ukraine, for fear of provoking Russia, Biden officials have recently warned that the United States could throw its weight behind a Ukrainian insurgency should Mr. Putin invade Ukraine.

And the deployment of thousands of additional American troops to NATO’s eastern flank, which includes Estonia, Latvia and Lithuania, Biden administration officials said, is exactly the scenario that Mr. Putin has wanted to avoid, as he has seen the western military alliance creep closer and closer to Russia’s own border.

In his news conference last week, Mr. Biden said he had cautioned Mr. Putin that a Russian invasion of Ukraine would prompt Washington to send more troops to the region.

“We’re going to actually increase troop presence in Poland, in Romania, etc., if in fact he moves,” Mr. Biden said. “They are part of NATO.”

During a phone call this month, Defense Secretary Lloyd J. Austin III warned his Russian counterpart, Sergey Shoygu, that a Russian incursion into Ukraine would most likely result in the exact troop buildup that Mr. Biden is now considering.

At the time of the phone call — Jan. 6 — the Biden administration was still trying to be more restrained in its stance on Ukraine. But after unsuccessful talks between Mr. Blinken and the Russian foreign minister, Sergey V. Lavrov, on Friday, the administration is eying a more muscular posture, including not only diplomatic options like sanctions, but military options like increasing military support to Ukrainian forces and deploying American troops to the region.

“This is clearly in response to the sudden stationing of Russian forces in Belarus, on the border, essentially, with NATO,” said Evelyn Farkas, the top Pentagon official for Russia and Ukraine during the Obama administration. “There is no way that NATO could not reply to such a sudden military move in this political context. The Kremlin needs to understand that they are only escalating the situation with all of these deployments and increasing the danger to all parties, including themselves.”

Another former top Pentagon official for Russia policy, Jim Townsend, said the administration’s proposal did not go far enough.

“It’s too little too late to deter Putin,” Mr. Townsend said in an email. “If the Russians do invade Ukraine in a few weeks, those 5,000 should be just a down payment for a much larger U.S. and allied force presence. Western Europe should once again be an armed camp.”

During the meeting at Camp David, Mr. Austin and Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, appeared by video from the Pentagon and from General Milley’s quarters, where he has been quarantining since he tested positive for the coronavirus. Officials said that if Mr. Biden approved the deployment, some of the troops would come from the United States, while others would move from other parts of Europe to the more vulnerable countries on NATO’s eastern flank.

American officials did not describe in detail the ground troop reinforcements under review, but current and former commanders said they should include more air defense, engineering, logistics and artillery forces.

Besides the troops, Mr. Biden could also approve sending additional aircraft to the region.

Representative Michael McCaul of Texas, the top Republican on the Foreign Affairs Committee, said on Sunday that the United States also needed to conduct more training in those NATO nations.

“We need joint exercises in Poland, the Baltic States, Romania, Bulgaria, to show Putin that we’re serious,” Mr. McCaul said on “Face the Nation.” “Right now, he doesn’t see we’re serious.”

According to Poland’s defense ministry, there are currently about 4,000 U.S. troops and 1,000 other NATO troops stationed in Poland. There are also about 4,000 NATO troops in the Baltic States.

The United States has been regularly flying Air Force RC-135 Rivet Joint electronic-eavesdropping planes over Ukraine since late December. The planes allow American intelligence operatives to listen to Russian ground commanders’ communications. The Air Force is also flying E-8 JSTARS ground-surveillance planes to track the Russian troop buildup and the movements of the forces.

The Biden administration is especially interested in any indication that Russia may deploy tactical nuclear weapons to the border, a move that Russian officials have suggested could be an option.

More than 150 U.S. military advisers are in Ukraine, trainers who have for years worked out of the training ground near Lviv, in the country’s west, far from the front lines. The current group includes Special Operations forces, mostly Army Green Berets, as well as National Guard trainers from Florida’s 53rd Infantry Brigade Combat Team.

Military advisers from about a dozen allied countries are also in Ukraine, U.S. officials said. Several NATO countries, including Britain, Canada, Lithuania and Poland, have regularly sent training forces to the country.

In the event of a full-scale Russian invasion, the United States intends to move its military trainers out of the country quickly. But it is possible that some Americans could stay to advise Ukrainian officials in Kyiv, the capital, or provide frontline support, a U.S. official said.