Martin Indyk, the chief US negotiator between the Israelis and Palestinians, resigned Friday in a further sign of the collapse of the peace process.
Less than a year after Secretary of State John Kerry tapped the veteran diplomat as part of a major US push for a peace deal, Indyk quit to return to his position at the Brookings Institution think tank.
Kerry credited Indyk with playing a "vital role" in the peace process, which the top US diplomat insisted was not dead.
"He'll continue to work for peace, and as we've all said many times, the United States remains committed not just to the cause of peace, but to resuming the process when the parties find a path back to serious negotiations," Kerry said in a statement.
"I am very grateful to Martin for his indefatigable efforts and creativity, and I look forward to continue working closely with him."
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Indyk, who was born in Britain and raised in Australia, formerly worked for the main pro-Israel lobby in Washington, the American Israel Public Affairs Committee, and took US citizenship in 1993 as he joined the administration of then-president Bill Clinton.
Indyk served twice as US ambassador to Israel -- from 1995-1997 and 2000-2001 -- and played a key role in Clinton's failed efforts to broker a Middle East peace settlement, including at the Camp David summit between then-Israeli prime minister Ehud Barak and Palestinian leader Yasser Arafat.
Kerry sought to revive Middle East diplomacy and last July coaxed Israeli Prime Minister Benjamin Netanyahu and Palestinian president Mahmud Abbas back to the negotiating table.
But Israel in April made a surprise announcement of plans for 700 new settlements and refused to free a last batch of Palestinian prisoners after earlier releases. Abbas in turn sought Palestinian membership in 15 UN conventions.
Israel voiced anger after an unnamed US official -- widely believed to be Indyk -- was quoted by the Yedioth Ahronoth newspaper as blaming Israel for the breakdown in talks and saying that Netanyahu "did not move more than an inch."