The US is sending a special radar-equipped warship to the Mediterranean Sea next week, in the first step to develop a broad anti-ballistic missile system to protect Europe against a potential Iranian nuclear threat, the Pentagon said.
The move marks the first of the Obama administration's four-phase plan to put land and sea-based radars and interceptors in several European locations during the next decade.
Endorsed by Nato during a summit in Lisbon last year, the missile shield has triggered opposition from Russia and set off lengthy negotiations over the future expanded ability to shoot down ballistic missiles in the region.
John Plumb, principal director for Pentagon nuclear and missile defence policy, said the USS Monterey would leave Norfolk, Virginia, next week, heading to a six-month deployment in the Mediterranean. The ship's mission, he said, would lay the groundwork for the unfolding missile defence plan there.
"Here is our first concrete demonstration of our commitment to the missile defence of our deployed forces, allies and partners in Europe," Mr Plumb said. "We said we were going to do it, and now we're doing it."
Under the plan laid out by the Obama administration in 2009, the missile shield would begin with ship-based anti-missile interceptors and radars. Later this year, the US plans to add land-based radars in southern Europe.
Mr Plumb said officials were still having talks with several nations, but the exact location for those radars had not yet been determined.
In phase two, land-based interceptors would be deployed in Romania in 2015, followed by the placement of interceptors in Poland in 2018. Each phase calls for a more sophisticated and capable interceptor, culminating at the end of the decade with the deployment of the last version planned as of now.
The goal is to protect Nato nations against medium-range missile attacks, with the focus being the growing nuclear threat from Iran.
The USS Monterey is a guided missile cruiser equipped with a sophisticated Aegis radar system. Other ballistic missile defence capable ships have been deployed by the US Navy to the Mediterranean since 2009, but this is the first to go under the new European defence plan.
The Bush administration first proposed stationing 10 interceptor missiles in Poland and an advanced radar in the Czech Republic. But Russia objected angrily and warned that it would station its own missiles close to Poland if the plan went ahead.
In September 2009, the US government shelved that plan and offered the new, reconfigured phased programme.
Since then, US and Russian officials have been struggling to find common ground that also would allay Moscow's fears that the system could target Russian warheads or undermine their deterrence strategy.
US officials are looking for ways to co-operate with Russia, including the possibility of combining sensors and sharing data. A Russian argument to have joint control over the missile defence system has been rebuffed by the United States.