By Sue Pleming

WASHINGTON, July 17 (Reuters) - He helped steer talks with Libya that led to a resumption of U.S. ties with Tripoli after nearly a quarter century and now senior diplomat William Burns is Washington's main interlocutor with Iran.

Burns, who as career ambassador holds the highest rank in the U.S. foreign service, will represent Washington in nuclear talks with Iran on Saturday, a sharp departure from U.S. policy that could be a launch point to reduced tensions.

Burns was involved in talks that led Libya to renounce terrorism and give up weapons of mass destruction in 2003, resulting in a thaw in relations and full diplomatic ties that were ultimately restored in May 2006.

"We are trying to make clear to Iran and its people what they stand to gain if they change course," Burns told Congress last week of U.S. efforts with Iran.

In March 2004, Burns, who was then Assistant Secretary of State for Near Eastern affairs, met Muammar Gaddafi, the highest-level U.S. diplomatic contact with Libya in 24 years, after Tripoli said it would renounce its weapons of mass destruction.

Fellow diplomats point to Burns' experience in such tricky negotiations and say they will serve him well in dealing with the Iranians, with whom Washington broke off ties shortly after the Iranian revolution of 1979.

"Bill Burns is probably and arguably the most respected and effective U.S. diplomat. Period. He is universally acclaimed in the region and within the department and by Republican and Democratic administrations," said Shibley Telhami, a Middle East expert with the University of Maryland.

Burns' mandate is to listen and not negotiate in Saturday's talks, but if Iran suspends its sensitive nuclear work, then the United States has promised to join full-blown negotiations.

QUIET, STEADY STYLE

A soft-spoken person, Burns, 52, joined the foreign service in 1982. His most recent post was U.S. ambassador to Russia from 2005 until he was appointed in May as Under Secretary of State for Political Affairs, replacing Nicholas Burns, not a relation. He is the third most senior official in the department after Rice and her deputy.

Burns was U.S. ambassador during a rocky time in U.S.-Russian relations, and fellow diplomats says his quiet, steady style will be an asset in dealing with Iran.

As a permanent member of the U.N. Security Council, Russia is one of the main players tackling Iran's nuclear dossier but Moscow has also been one of the most resistant among major powers to impose sanctions on Iran.

Burns' knowledge of Russia and the language is seen by many diplomats as an advantage in trying to convince Moscow to be tough on Iran in negotiations.

Aside from Russian, Burns also speaks Arabic and French. He was ambassador to Jordan from 1998 to 2001.

Burns has a BA in history from LaSalle University in Philadelphia, and degrees in international relations from Oxford University where he studied as a Marshall Scholar.

He has received two presidential distinguished service awards and was in 1994 on Time Magazine's list of the "50 Most Promising American Leaders under Age 40" and on TIME's list of "100 Young Global Leaders."

Burns is married to Lisa Carty and has two daughters. (Editing by Philip Barbara)

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