"They are directed toward rogue nations' capabilities not obviously a sophisticated ballistic missile fleet such as the Russians have," General Henry Obering, the US Missile Defence Agency chief, said on Thursday.
He said that that the system would be designed to intercept missiles from Iran.
Earlier in the week, Sergei Ivanov, the Russian defence minister, rejected Washington's reassurances, saying Iranian missiles could not reach Europe.
|"The creation of a US anti-missile base in Europe can only be viewed as a considerable change in the configuration of the American military presence in Europe"|
Russian foreign ministry
Russian opposition to the the plans reflects concerns about the United States' growing influence - military and political - in former Soviet nations that have joined Nato and the European Union.
"The creation of a US anti-missile base in Europe can only be viewed as a considerable change in the configuration of the American military presence in Europe," the Russian foreign ministry statement said.
"The presence of such a base near our borders will be a factor we will have to take into account when planning the next military and political steps, building national defence."
The US state department announced on Sunday that Poland and the Czech Republic had agreed to begin detailed talks on allowing their territory to be used for the system.
The European-based system would form part of part of a "shield" against ballistic missiles that the US says could be tipped with chemical, germ or nuclear weapons. Up to 25 ground-based interceptors are due to be installed in Alaska and California, primarily to counter a potential threat from North Korea.