The United States announced Friday it will deploy 1,000 troops to Poland as part of broader NATO efforts to reassure former Communist eastern member states fearful of a more assertive Russia.
Speaking at a NATO summit in Warsaw, President Barack Obama said the troops would serve "shoulder to shoulder" with Polish forces.
They are expected to conduct frequent training missions and will be "mechanized", meaning they would have regular infantry equipment including armored personnel carriers.
Britain said earlier this week it would commit 650 troops to a separate battalion, and fellow NATO allies Germany and Canada have also pledged to stand up their own units.
The troops will rotate through Poland plus the three small Baltic states of Estonia, Latvia and Lithuania, acting as a tripwire to deter any Russian adventurism.
They are backed up by a "Spearhead Force" -- officially the "Very High Readiness Joint Task Force" -- which numbers about 5,000 troops ready to move within a couple of days.
'Feeling of intimidation'
NATO has been working to prevent a repeat of Russia's Ukraine intervention and annexation of Crimea in 2014, with former Soviet-bloc members anxious they could be vulnerable should Moscow attempt additional land grabs.
NATO has mounted a series of exercises, especially in the eastern member states, to test readiness levels and reassure nervous allies, and it has also deployed extra aircraft to boost air policing, especially over the Baltic states.
Further south, NATO is increasingly focusing on alliance members Romania and Bulgaria as they cast a wary eye across the Black Sea, where the Russians are building up their military presence.
NATO has announced plans to set up a similar reassurance force in Romania.
"We are seeing in the Black Sea increasingly a feeling of intimidation," a senior US defense official said.
Aside from the four NATO battalions, the United States is separately pumping more military resources into Europe, this year pledging $3.4 billion in "reassurance" spending.
The Pentagon has separately announced the deployment from next year of an armored brigade of 4,200 troops and Obama said Friday this unit's headquarters will be in Poland.
"In other words, Poland will be seeing an increase in NATO and American personnel and in the most modern military equipment," Obama said.
Obama did not provide details on where the US troops comprising the NATO battalion would come from, or where they would be stationed.
The United States is also building a missile defence system in Europe, which NATO was due to take control of as early as Friday, the US defense official said.
"Unless there's some last-minute hiccup... later this evening, NATO will move into command and control position," the official said.
Obama's announcement came as the Atlantic alliance began a two-day summit in the Polish capital billed as one of the most important such gatherings since the end of the Cold War.
NATO leaders also discussed the longstanding issue of a 2014 decision to reverse years of spending cuts and require countries to commit two percent of annual economic output to defense.
Progress since then has been patchy, with only five of the 28 member states meeting the target at a time of austerity.