WASHINGTON — His two-year mission unfulfilled, Sen. George Mitchell announced his resignation today as the Obama administration’s special envoy to the Mideast at a time of turmoil in the region and fruitless attempts at Israeli-Palestinian peace talks.

President Barack Obama, accepting the resignation, called Mitchell “a tireless advocate for peace.”

In a two-paragraph letter to Obama, Mitchell said that he took the diplomatic job intending to only serve two years. “I strongly support your vision of comprehensive peace in the Middle East and thank you for giving me the opportunity to be part of your administration,” Mitchell wrote.

Mitchell’s resignation comes at a critical time for the Middle East, which is embroiled in uprisings, and the Israeli-Palestinian peace process, which has been moribund since last September and is now further complicated by an agreement between Palestinian factions to share power.

“This is, I think, a moment in time when the parties are considering how best to proceed – given the events in the Middle East which have created a great deal of uncertainty and anxiety throughout the region, including among the Israelis and Palestinians,” Mitchell said in an interview this afternoon with MaineToday Media. “So at this moment in time, it seemed about right.”

Obama will deliver a speech next Thursday at the State Department about his administration’s views of developments in the region, ahead of a visit to Washington by Israeli Prime Minister Benjamin Netanyahu. Jordan’s King Abdullah II also will travel to Washington next week. Michell’s resignation will be effective May 20 — the same day Netanyahu visits the White House

David Hale, Mitchell’s deputy, will serve as acting envoy, Obama said in a statement.

In a telephone interview today with MaineToday Media, Obama said: “George is by any measure one of the finest public servants our nation has ever had.” He aded that Mitchell is also “a good friend.”

White House spokesman Jay Carney said the administration remains focused on the Middle East peace process.

“The president’s commitment remains as firm as it was when he took office,” Carney said. “This is a hard issue, an extraordinary hard issue.”

Since his appointment on Obama’s second full day in office in January 2009, Mitchell, 77, had spent much of his time shuttling between the Israelis, Palestinians and friendly Arab states in a bid to restart long-stalled peace talks that would create an independent Palestinian state. But in recent months, particularly after the upheaval in Arab countries that ousted longtime U.S. ally and key peace partner Hosni Mubarak from power in Egypt, his activity had slowed markedly.

Mitchell has led a long career as politician, businessman, congressional investigator and international mediator.

Upon being announced as the administration’s point man for Mideast negotiations, he recalled his role in producing Northern Ireland’s Good Friday peace accord in 1998.

“We had 700 days of failure and one day of success,” he said. “For most of the time, progress was nonexistent or very slow.”

Mitchell believed his patience would serve him well in the Arab-Israeli conflict and its constant forward and backward steps. Speaking of the Northern Ireland conflict, he added: “I formed the conviction that there is no such thing as a conflict that can’t be ended. Conflicts are created, conducted and sustained by human beings. They can be ended by human beings.”

Mitchell served in the Senate as a Democrat from Maine from 1980 to 1995, the final six years as majority leader. In 2000-01, he headed a fact-finding committee on Mideast violence that called for commitments by Israel and the Palestinian Authority to immediately and unconditionally end their fighting. The panel urged a cooling-off period and other steps toward peace, but it did not lead to lasting results.

The April 2001 Mitchell report asked Israel to freeze settlements in the West Bank and called on the Palestinians to prevent gunmen in Palestinian-populated areas from firing on Israeli towns and cities. The settlements, as well as Israeli concern over rocket and other attacks on its soil, remain sticking points today.

U.S. Sen. Olympia J. Snowe said in a statement today that “Senator Mitchell took on an incredibly difficult task during an increasingly challenging time in the Middle East.  I commend his work and appreciate his commitment to the Middle East peace process.  His hallmark facility for skilled diplomacy and deft negotiation is, and continues to be, an asset to our nation and the world.”

Mitchell also led the 2007 investigation into the use of performance-enhancing drugs in major league baseball. Before that, he was chairman of The Walt Disney Co. from 2004-2006.