A US deployment of ballistic missile defence systems in Romania and plans to place more defence systems in Poland violate an existing arms treaty, Russia's foreign ministry says.
The US switched on an $US800 million ($A1.1 billion) missile shield in Romania nearly a year ago and was planning to create another site in Poland, seeing it as vital to defend itself and Europe from so-called rogue states.
In 2016, the Kremlin said it was aimed at blunting its own nuclear arsenal.
The foreign ministry said on Saturday the plans violated the Intermediate-Range Nuclear Force Treaty (INFT), signed by Washington and the Soviet Union in the late 1980s in an attempt to eliminate nuclear and conventional short-and intermediate range missiles.
"The undeniable fact is that this is a gross violation of the INFT obligations," the ministry said on its website.
