Russia has vowed to respond to the signing of a US-Poland missile defence deal and "not only by diplomatic protests".
Moscow warned on Wednesday that the pact to station 10 US interceptor missiles just 180km from Russia's westernmost frontier would fuel a new arms race in Europe.
"Such actions create mistrust and spur an arms race on the continent and beyond its borders," a Russian foreign ministry statement on Wednesday said.
The response came despite an assurance from Condoleezza Rice, the US secretary of state, that Russia had nothing to fear from the missile pact.
She also denied the possibility of a new Cold War.
"Missile defence of course is aimed at no one. It is in our defence that we do this," said Rice, adding later: "It is not aimed in any way at Russia."
The US plans to base 10 interceptor missiles in Poland and a radar facility in the neighbouring Czech Republic between 2011 and 2013.