A £552m missile defence system has been switched on by the United States in Romania despite Russian warnings that the West is threatening the peace in central Europe.
Located at the remote Deveselu air base, the ballistic missile defence site is capable of shooting down rockets from so-called rogue states, which Washington says could one day reach major European cities.
"Iran continues to develop, test and deploy a full range of ballistic missile capabilities and those capabilities are increasing in range and accuracy," said Frank Rose, deputy US assistant secretary of state for arms control. "Iran's systems can reach into parts of Europe, including Romania."
Yet Russia has expressed alarm at such a show of force by its Cold War rival in formerly communist-ruled Eastern Europe.
Moscow says the US led alliance is trying to encircle it close to the strategically important Black Sea, home to a Russian naval fleet and where NATO is also considering increasing patrols.
At a cost of billions of dollars, the missile defence umbrella relies on radars to detect a ballistic missile launch into space.
Sensors then measure the rocket's trajectory and destroy it in space before it re-enters the earth's atmosphere. The interceptors can be fired from ships or ground sites.
The Kremlin says the shield's aim is to neutralise Moscow's nuclear arsenal long enough for the United States to strike Russia in the event of war, although Washington denies this.
"We are not meddling in anything that could be perceived as potentially destabilising," said Douglas Lute, US envoy to NATO.
The United States says Russia has ballistic missiles, breaching a treaty that agreed the two powers must not develop and deploy missiles with a range of 500 km to 5,500 km. The United States declared Russia in non-compliance of the treaty in July 2014.
Ballistic missiles differ from cruise missiles because they leave the earth's atmosphere and travel distances of up to 3,000 km.
The issue remains highly sensitive as the United States does not want to give any impression it would be able to shoot down Russian ballistic missiles that were carrying nuclear warheads, which is what Russia fears.