(Recasts throughout)

By David Brunnstrom

BRUSSELS, March 1 (Reuters) - The United States vowed on Thursday to press ahead with a plan for a missile defence shield in eastern Europe to guard against attack from Iran, with or without the approval of NATO allies.

The head of the Pentagon's Missile Defence Agency said Washington may want to put radar even closer to Russia -- which opposes the plan -- in the Caucasus, in addition to planned sites in ex-Warsaw Pact members Poland and the Czech Republic.

When asked whether the plan needed approval from NATO's 26 members, Air Force Lt. Gen. Henry Obering said:

"It's important that we get the understanding and what I would consider to be as much partnering as we can do with our NATO allies. We are not looking for approval per se."

Iran, accused by the West of pursuing nuclear weapons, already had missiles that could reach some NATO allies and by 2015 could threaten the United States, he said.

"We do believe there's an urgency to the threat," he said.

Russian President Vladimir Putin has denounced the U.S. plan as a threat to national security. Obering expressed surprise at the reaction and said the project was purely defensive.

He said Washington was willing to have further talks and that the Russians could inspect the interceptor missile battery in Poland and the Czech radar site if those countries agreed.

"We want to be open and transparent," he said. He added it would be useful, but not essential, to place another radar installation somewhere in the Caucasus.

EU foreign policy chief Javier Solana said it was "questionable" whether a missile threat existed to Europe and it was up to European countries to decide whether to take part.

"Every country in the EU is free to accept whatever they want to accept ... We are not as Europeans considering to establish such a mechanism of that type," he said from the sidelines of an EU defence ministers' meeting.

Obering said he expected Boeing Co <BA.N> to lead the interceptor site in Poland, which would cost $2-2.5 billion, $900 million of which could go to contracts to local firms.

The Czech radar site would cost around $500 million, some $150-200 million of which could go locally, he said.

If a deal is reached this year, building at the interceptor site could begin in 2008, the first interceptor missiles positioned in 2011, and the programme competed in 2013.

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