By Robin Emmott

DEVESELU, Romania (Reuters) - The United States switched on an $800 million missile shield in Romania on Thursday that it sees as vital to defend itself and Europe from so-called rogue states but the Kremlin says is aimed at blunting its own nuclear arsenal.

To the music of military bands at the remote Deveselu air base, senior U.S. and NATO officials declared operational the ballistic missile defense site, which 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 U.S. Deputy Defense Secretary Robert Work, standing in front of the shield's massive gray concrete housing that was adorned with a U.S. flag.

Despite Washington's plans to continue to develop the capabilities of its system, Work said the shield would not be used against any future Russian missile threat. "There are no plans at all to do that," he told a news conference.

Before the ceremony, Frank Rose, deputy U.S. 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 defense 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 U.S. air base in Germany.

Russia is incensed at such of show of force by its Cold War rival in formerly communist-ruled eastern Europe. Moscow says the U.S.-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.

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"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.

Russian President Vladimir Putin's office said Moscow also doubted NATO's stated aim of protecting the alliance against Iranian rockets following the historic nuclear deal with Tehran and world powers last year that Russia helped to negotiate.

"The situation with Iran has changed dramatically," Putin's spokesman Dmitry Peskov said.

Joe Cirincione, an American nuclear expert who is president of Ploughshares Fund, a global security organization, told reporters in Geneva that the shield should be scrapped.

"It was designed to protect Europe from a missile from, well, the only country we were afraid of was Iran. The system was designed to protect against an Iranian nuclear missile. There is not going to be an Iranian nuclear missile for at least 20 years. There is no reason to continue with that program."

RETALIATION

The readying of the shield also comes as NATO prepares a new deterrent in Poland and the Baltics following Russia's 2014 annexation of Crimea. In response, Russia is reinforcing its western and southern flanks with three new divisions.

Poland is concerned Russia may retaliate further by announcing the deployment of nuclear weapons to its enclave of Kaliningrad, located between Poland and Lithuania. Russia has stationed anti-aircraft and anti-ship missiles there, able to cover huge areas and complicate NATO's ability to move around.

The Kremlin says the shield's aim is to neutralize Moscow's nuclear arsenal long enough for the United States to strike Russia in the event of war. Washington and NATO deny that.

"Missile defense ... does not undermine or weaken Russia's strategic nuclear deterrent," NATO Secretary-General Jens Stoltenberg said at the Deveselu base.

However, Douglas Lute, the United States' envoy to NATO, said NATO would press ahead with NATO's biggest modernization since the Cold War. "We are deploying at sea, on the ground and in the air across the eastern flanks of the alliance ... to deter any aggressor," Lute said.

At a cost of billions of dollars, the missile defense 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 Romanian shield, which is modeled on the United States' so-called Aegis ships, was first assembled in New Jersey and then transferred to the Deveselu base in containers.

While U.S. and NATO officials are adamant that the shield is designed to counter threats from the Middle East and not Russia, they remained vague on whether the radars and interceptors could be reconfigured to defend against Russia in a conflict.

The United States says Russia has ballistic missiles, in breach of a treaty that agreed the two powers must not develop and deploy missiles with a range of 500 km (310.69 miles) to 5,500 km. The United States declared Russia in non-compliance of the treaty in July 2014.

The issue remains sensitive because the United States does not want to give the impression it would be able to shoot down Russian ballistic missiles that were carrying nuclear warheads, which is what Russia fears.

(Additional reporting by Jack Stubbs, Andrew Osborn and Maria Tsvetkova in Moscow and Tom Miles in Geneva; Editing by Tom Heneghan and Andrew Heavens)