WASHINGTON (Reuters) - Outgoing Israeli Defense Minister Ehud Barak will receive the highest award he could be given by a U.S. secretary of defense when he visits the Pentagon on Thursday, three days after announcing his exit from political life next year.

The 70-year-old Barak, a leading strategist in confronting Iran over its nuclear program who has also served as Israel's prime minister and armed forces chief, has been a regular visitor to the Pentagon in recent years as tensions with Tehran simmer.

Defense Secretary Leon Panetta has known the Israeli leader since President Bill Clinton's administration, when Panetta was chief of staff and when Barak served in roles including foreign minister. Barak and Panetta speak regularly, with three conversations alone during the crisis in Gaza this month.

"He's been an important partner of the U.S. for a long time," one U.S. defense official told Reuters, adding he will receive the Department of Defense Medal for Distinguished Public Service.

Should Barak's resignation prove permanent, his successor could come from the ranks of right-wing Likud party of Prime Minister Benjamin Netanyahu, who has been seen at odds with Washington over the best way to handle Iran.

Ex-Israeli general Moshe Yaalon, who has talked tough on Iran but is more circumspect among Netanyahu's advisors, is a possible candidate to succeed the more moderate Barak. He is the minister of strategic affairs and is a former chief of staff of the Israeli defense forces.

There has been speculation that Barak might even be replaced by the current foreign minister, Avigdor Lieberman, the Likud's more hawkish coalition partner.

"The fact is that none of us know," said Anthony Cordesman, at the Washington-based Center for Strategic and International Studies.

The Pentagon announced that Panetta and Barak will address a news conference at 2:30 p.m. Eastern Standard Time (1730 GMT).

(Reporting by Phil Stewart, editing by Stacey Joyce)