Washington has voiced alarm at Russia's admission it has moved nuclear-capable Iskander missiles closer to EU borders in response to the NATO air defence shield.
The Russian missile has a range of 500 kilometres and could be used to attack ground-based radar and interceptors of the new NATO shield.
Moscow's announcement on Monday prompted concern from the US as well as Poland and three Baltic states, Estonia, Lithuania and Latvia.
"We've urged Russia to take no steps to destabilise the region," State Department deputy spokeswoman Marie Harf said, adding the US had also passed on its neighbours' concerns.
Poland's foreign ministry said the deployment was disturbing.
"This is a matter for NATO and we can expect possible consultations and action at the NATO and EU level," it said in a statement.
Latvia Defence Minister Artis Pabriks said "several Baltic cities" were threatened by the move.
He told the Baltic News Service it was alarming and was changing the balance of power in the region.
Germany's Bild newspaper reported over the weekend that Russia had deployed about 10 Iskander systems in its Kaliningrad exclave, wedged between Poland and Lithuania, at some point in the past year.
A top Russian defence official said in response that several Iskander batteries had been stationed in Russia's western military district, a region that includes the exclave and borders the three Baltic nations that were once a part of the USSR.
"Iskander operational-tactical missile systems have indeed been commissioned," defence ministry spokesman Igor Konashenkov told Russian news agencies.
He added that Russia's deployment "does not violate any international treaties or agreements" and should not be subject to protests from the West.
The Kremlin warned in 2011 that it could station the short- and medium-range ballistic missiles along the EU's eastern frontier in response to NATO's missile defence program.
Both the US and NATO have argued the shield is not aimed at Russia but is designed to protect the West from potential threats from so-called "rogue states".
But Moscow fears the system, which includes missile-positioning satellites, may one day target Russia.
Russian Foreign Minister Sergei Lavrov has said a deal with Tehran to curb its nuclear program would make the NATO shield unnecessary.
But US Defence Secretary Chuck Hagel told his Russian counterpart Sergei Shoigu on Monday that a preliminary nuclear deal with Iran "does not eliminate the need" for the NATO shield.
"Hagel stressed that US and NATO missile defence efforts pose no threat to Russia and urged that both sides continue consultations on future missile plans in Europe," the Pentagon said in a statement.
"The Iskander is a type of weapon that could influence the military and political situations in certain regions of the world," Russia's state-owned ITAR-TASS news agency said.
But military analysts said cash constraints had prevented the armed forces from deploying as many Iskander systems as originally hoped.
It was estimated that 60 Iskanders would be in operation by 2015 - far too few to pose a serious threat to the West.
