Around 1,700 US service members, mainly from the 82nd Airborne Division, will deploy from Fort Bragg, North Carolina to Poland, while a Stryker squadron of about 1,000 troops based in Vilseck, Germany will be sent to Romania, a spokesman for the US Defense Department said, as cited by the Reuters news agency.
Three hundred other service members will move from Fort Bragg to Germany, the Pentagon spokesman, John Kirby, told a news conference.
The aim is to send a "strong signal" to Russian President Vladimir Putin "and frankly, to the world, that NATO matters to the United States and it matters to our allies," Kirby told reporters.
"We worked closely with our Polish and German allies to set the stage for these movements, and we appreciate their support," he also said.
He added: "These are not permanent moves. They respond to current conditions. We will adjust our posture as those conditions evolve."
Kirby's announcement came after broadcaster CNN reported earlier on Wednesday that US President Joe Biden had formally approved additional military deployments to eastern Europe, with plans for the soldiers to deploy "in the coming days."
CNN cited officials as saying that the deployments would be a show of support to NATO allies feeling threatened by Russia's military moves near Ukraine.
Polish Defence Minister Mariusz Błaszczak said in a tweet that the planned deployment of 1,700 additional US troops to Poland was "a strong signal of solidarity in response to the situation in Ukraine."
Russia's military buildup near Ukraine has in recent weeks raised fears in the West that Moscow may be preparing for a new invasion of the country.
Moscow has denied plans for an assault but says it could take unspecified military measures if its security demands are not met, including a promise by NATO never to admit Kyiv, Reuters reported.
Russia annexed the Crimea peninsula from Ukraine in 2014 and then fomented a separatist conflict in that country's eastern Donbas region, leading to a wave of EU and US sanctions against Moscow and Russian officials, Poland's PAP news agency reported.
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Source: Reuters, CNN, defense.gov, PAP