Updated

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

President 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.

Mitchell's resignation appears to have been timed to match Obama's increased public focus on the region. The president 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. Obama also will play host to Jordan's King Abdullah II on Tuesday. Mitchell's last day will be effective May 20 -- the same day Netanyahu visits the White House.

The White House was also looking to schedule a speech by Obama to the American Israel Public Affairs Committee before he leaves for Europe May 22, officials said.

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

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

"The president's commitment remains as firm as it was when he took office," Carney said. "This is a hard issue, an extraordinarily hard issue."

On his second full day in office in January 2009, Obama appointed Mitchell to the special envoy's post amid much fanfare. The former Democratic senator from Maine, who rose to be Senate majority leader, had established his credentials as an international mediator by helping broker peace in Northern Ireland. As such, he brought an outsize profile to one of the most intractable diplomatic undertakings.

Since his appointment, Mitchell, 77, has 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.

Partisans blamed each other for Mitchell's inability to bring the sides together.

Nimer Hamad, a senior adviser to Palestinian President Mahmoud Abbas, told the AP that Mitchell's job had been made more difficult by Israeli intransigence.

"Mitchell hasn't been in the region in three months," Hamad said. "Whether he resigns or not, it's clear that Mitchell wasn't in the region because he didn't see the possibility of being a mediator between two sides where one of them is not responsive."

AIPAC, the pro-Israel lobby, issued a statement criticizing Palestinian Authority President Mahmoud Abbas, citing his "unwillingness to negotiate directly with his Israeli counterpart without preconditions." AIPAC also denounced Abbas's secular Fatah movement for singing a reconciliation deal with Hamas, the Islamic militant group that governs the Gaza Strip, rather than make peace with Israel.

Jon Alterman, director of the Middle East program at the Center for Strategic and International Studies, said Mitchell's task held greater hope at the time of his appointment.

"But the way the politics worked out, you have an Israeli government that is very skeptical about the ability of negotiating with the Palestinian Authority," Alterman said. "And you have you have a Palestinian Authority where the internal politics are increasingly fraught.

"So it's hard to find a political consensus either among the Israelis or the Palestinians to move forward on the kinds of negotiations that George Mitchell was appointed to facilitate."

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."

Israeli analyst Yossi Alpher, former director of the Jaffee Center for Strategic Studies at Tel Aviv University, said Mitchell's departure could signal a different approach by the Obama administration.

"His methods just didn't work here," Alpher said. "The Northern Ireland method of listen, listen and listen doesn't work here."

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.

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.