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U.S. President Barack Obama promised on Tuesday to beef up military support for eastern European members of the NATO alliance who fear they could be next in the firing line after the Kremlin's intervention in Ukraine.

Under fire from critics at home who say his leadership on the world stage has not been muscular enough, Obama unveiled plans to spend up to $1 billion in supporting and training the armed forces of NATO states on Russia's borders.

"We'll increase the number of American personnel, Army and Air Force units, continuously rotating through allied countries in central and eastern Europe. And we will be stepping up our partnerships with friends like Ukraine, Moldova and Georgia, as they provide for their own defense," Obama said.

The White House also said it would review permanent troop deployments in Europe in the light of the Ukraine crisis -- though that fell short of a firm commitment to put troops on the ground that