US plans to expand its embryonic missile defence shield to the Czech Republic and Poland are an "an obvious threat", the Russian military says.
The US has had mixed results from its missile tests
Poland has confirmed the US wants to negotiate the use of its territory to build part of its missile defence base.
On Sunday, the US asked permission from the Czech Republic and received the backing of Czech PM Mirek Topolanek.
Washington says it needs interceptor missiles in Europe to stop attacks by states like Iran or North Korea.
It hopes to build a radar station in the Czech Republic and to site interceptors in Poland.
Poland's Deputy Foreign Minister Witold Waszczykowski confirmed that Washington has approached Warsaw over the project and said: "We are now waiting for firm proposals."
But Moscow insists that the installation of US missiles in countries close to its western border would change the strategic balance in Europe.
Lt Gen Vladimir Popovkin, commander of Russia's space forces, said Moscow would interpret the move as a military threat.
"Our analysis shows that the deployment of a radar station in the Czech Republic and a counter-missile position in Poland are an obvious threat to us.
"It is very doubtful that elements of the national US missile defence system in eastern Europe were aimed at Iranian missiles, as has been stated," he said.
Moscow has warned of "negative consequences" if Prague agrees to host the missile system.
Mr Topolanek, the Czech prime minister, has welcomed the US request.
"We are convinced that a possible deployment of the radar station on our territory is in our interest," he said at the weekend.
"It will increase security of the Czech Republic and Europe."
However, Mr Topolanek could face a struggle having the plans approved by both houses of the country's parliament.
His three-party, centre-right governing coalition recently won a vote of confidence, but controls just 100 of 200 seats in the lower house.
There is domestic opposition to the scheme in the Czech Republic, with reports that 200 protesters rallied against the missile defence plans in Prague on Monday.
The US has already built missile interceptor sites in Alaska and in California, but says it needs to expand into Europe to counter growing threats from further afield.