WASHINGTON, May 4 (Reuters) - The United States has agreed for U.S. and Russian defense and foreign ministers to meet to try allay Russian concerns about U.S. missile defense plans in Eastern Europe, a senior U.S. official said on Friday.

U.S. Assistant Secretary of State Dan Fried said there had been discussion of holding so-called "2+2" talks among the four ministers in September or early in the fall but no date had been set.

Washington has angered Russia and unsettled some European allies with a plan to deploy 10 missile interceptors in Poland and radar in the Czech Republic from 2012 to help shield Europe from possible missile attack by nations such as Iran.

While European concerns about the missile shield appear to be easing, Russia has yet to be persuaded by U.S. arguments that the system is no threat to its nuclear deterrent and has so far rebuffed U.S. invitations to cooperate on the system.

Russian President Vladimir Putin has frozen commitments under a post-Cold War treaty on conventional force levels in Europe in protest.

Fried said he was hopeful that, over time, Russian concerns could be addressed and said one venue for doing so could be talks among the defense and foreign minsters.

"We have agreed to a Russian suggestion that the secretaries of defense and state meet with their Russian counterparts," Fried told reporters, saying U.S. and Russian national security advisers might also be included.

U.S. Secretary of State Condoleezza Rice last week said it was "ludicrous" to suggest the system the United States plans to build in Poland and the Czech Republic would threaten Russia's nuclear deterrent of thousands of warheads.

Fried said he did not know exactly what underpins Russian objections but suggested they may reflect a misunderstanding of the proposed system, the fear that it could be expanded to threaten Russia's nuclear deterrent or a desire to stir up misgivings in parts of Europe about the missile shield.

"Whatever the explanation, and as I have said we don't know, our answer is to offer cooperation with the Russians ... and be completely transparent in everything we do," he said.

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