(Adds comments by Pentagon spokesman, reaction from Baltic states, details on deployments)
By Phil Stewart and David Alexander
WASHINGTON, April 22 (Reuters) - The United States is sending about 600 soldiers to Poland and the three Baltic states for infantry exercises, the Pentagon said on Tuesday, one of its highest-profile steps yet to reassure NATO allies after Russia's seizure of Crimea.
Groups of 150 soldiers will be sent to Poland, Estonia, Lithuania and Latvia in rounds of month-long exercises in each country in what the Pentagon calls a "persistent rotational presence" that stops short of permanently basing troops.
Rear Admiral John Kirby, a top Defense Department spokesman, said the deployments sent a strong message to NATO allies about U.S. commitments to the alliance following events in Ukraine.
"If there's a message to Moscow, it is the same exact message that we take our obligations very, very seriously on the continent of Europe," Kirby said, leaving open the possibility the drills, set to last through 2014, could continue into 2015.
The United States and NATO have made clear they will not intervene militarily in Ukraine, which is not a NATO member, despite Russia's annexation of Ukraine's Crimea region and a buildup of Russian forces on the Ukrainian border.
They are instead focusing on temporarily boosting their presence in eastern Europe in a drive to reassure allies, such as the former Soviet Republics in the Baltics, that NATO would protect them if they ever faced Russian aggression.
Baltic states Estonia, Latvia and Lithuania - with their own Russian-speaking minorities - have been increasingly worried that Russia's annexation of Crimea, partly on ethnic grounds, could herald destabilization in their own region by Moscow.
The Latvian government welcomed the announcement, calling it a "fast and practical response." Estonian Defense Minister Sven Mikser said the move "significantly increases the security of Estonia."
Poland, which itself emerged from Soviet-imposed communism only in 1989, has called for a strong NATO troop presence in eastern Europe.
Still, Russia says deployment of significant NATO forces in eastern European countries close to Russia would violate the 1997 Founding Act, a cooperation agreement between Moscow and the alliance.
Lithuanian President Dalia Grybauskaite said the deployments would "strengthen our readiness for collective defense and will add to ensuring safety of our people."
Kirby said the first company-sized contingent of about 150 U.S. paratroopers from the U.S. Army's 173rd Infantry Brigade Combat Team based in Italy would arrive in Poland on Wednesday.
Additional companies will travel to Lithuania, Latvia and Estonia and would be in place by Monday, he said.
Beyond the troop deployments, the Pentagon also announced the return of the frigate USS Taylor to the Black Sea, just as the guided-missile destroyer USS Donald Cook prepares to depart.
A U.S. Navy official, speaking on condition of anonymity, said Cook arrived in the Black Sea on Tuesday and was expected to overlap with the Taylor for a couple of days. (Additional reporting by Andrius Sytas in Vilnius, David Mardiste in Tallinn and Aija Krutaine in Riga; Editing by Jason Szep and Mohammad Zargham)