Even before NATO officials announced plans on Wednesday to increase the alliance’s military strength in Eastern Europe, the allies had already stepped up the number of troops stationed in the region in response to Russia’s invasion of Ukraine.

About 40,000 troops are currently deployed under direct NATO command across Estonia, Latvia, Lithuania, Poland, Slovakia, Hungary, Romania and Bulgaria. That is a twofold increase since Russian troops entered Ukraine on Feb. 24.

The largest contingent is stationed in Poland, with 10,500 allied troops, on top of Poland’s 120,000 troops. The country’s battle group is led by the United States.

NATO has 30 members, 28 of them European. Across the alliance, NATO can count on nearly 3.5 million troops and other personnel.

About 100,000 U.S. troops are currently deployed in Europe, according to the alliance’s data.

Speaking at a news conference before Thursday’s NATO summit in Brussels, the group’s secretary general, Jens Stoltenberg, said that he expected the alliance to make a decision on doubling the number of battlegroups on its eastern flank.

That would mean deploying more NATO troops to Bulgaria, Hungary, Romania and Slovakia. He said the alliance would also increase support to Ukraine and to other partners at risk from Russian pressure.

“I expect leaders will agree to strengthen NATO’s posture in all domains, with major increases in the eastern part of the alliance on land, in the air and at sea,” Mr. Stoltenberg said.

President Vladimir V. Putin of Russia has said the eastward expansion of NATO and Ukraine’s desire to join the alliance represents a threat to Russia and justifies the invasion.