A U.S. 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 said in a statement on Saturday.

The United States switched on an $800 million 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 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.