Former US senator George Mitchell has resigned as President Barack Obama's special envoy to the Middle East amid turmoil in the region and fruitless attempts at rekindling Israeli-Palestinian peace talks.
In a two-paragraph letter to Mr Obama, Mr Mitchell said he took the diplomatic job intending to serve only 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," he wrote.
Mr Obama accepted the resignation from a man he called "a tireless advocate for peace".
Mr 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 in territory they control.
Mr Mitchell's resignation appears to have been timed to match Mr Obama's increased public focus on the region. The President will deliver a speech next Thursday at the US State Department about his administration's views of developments in the region, ahead of a visit to Washington by Israeli prime minister Benjamin Netanyahu.
The President will also play host to Jordan's King Abdullah II on Tuesday. Mr Mitchell's last day will be May 20 - the same day Mr Netanyahu visits the White House.
David Hale, Mr Mitchell's deputy, will serve as acting envoy, Mr Obama said.
White House spokesman Jay Carney said the administration remained focused on reviving Middle East peace negotiations. "The President's commitment remains as firm as it was when he took office," he said. "This is a hard issue, an extraordinarily hard issue."
On his second full day in office in January 2009, Mr Obama appointed Mr Mitchell to the special envoy's post amid much fanfare.