The United States has switched on an $US800 million ($A1.08 billion) missile shield in Romania.
It's a step it sees as vital to defend itself and Europe from so-called rogue states but which the Kremlin says is aimed at blunting its own nuclear arsenal.
To the music of military bands at the remote Deveselu air base, senior US and NATO officials on Thursday declared operational the ballistic missile defence site.
It is capable of shooting down rockets from countries such as Iran that Washington says could one day reach major European cities.
"As long as Iran continues to develop and deploy ballistic missiles, the United States will work with its allies to defend NATO," said US Deputy Defence Secretary Robert Work, standing in front of the shield's massive grey concrete housing that was adorned with a US flag.
Before the ceremony, Frank Rose, deputy US assistant secretary of state for arms control, warned that Iran's ballistic missiles can hit parts of Europe, including Romania.
When complete, the defensive umbrella will stretch from Greenland to the Azores.
On Friday, the United States will break ground on a final site in Poland due to be ready by late 2018, completing the defence line first proposed almost a decade ago.
The full shield also includes ships and radars across Europe.
It will be handed over to NATO in July, with command and control run from a US air base in Germany.
Russia is incensed 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.
"It is part of the military and political containment of Russia," Andrey Kelin, a senior Russian Foreign Ministry official, said on Thursday, the Interfax news agency reported.
"These decisions by NATO can only exacerbate an already difficult situation," he added, saying the move would hinder efforts to repair ties between Russia and the alliance.
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. Washington and NATO deny that.
