Veteran diplomat Ryan Crocker will step down this summer from his post as U.S. ambassador to Afghanistan for health reasons.
Crocker, 62, came out of retirement last July at the request of U.S. President Barack Obama. He originally had said that he expected to serve for two years in Kabul, after being the U.S. ambassador to Iraq from 2007 to 2009.
He was also previously Washington's ambassador to Lebanon, Pakistan, Kuwait, and Syria.
In a statement, U.S. State Department spokeswoman Victoria Nuland said Crocker's tenure "has been marked by enormous achievements."
She later told reporters: "I think his heart will always be a little bit in Afghanistan. But, obviously, he's not leaving yet. He's going to stay on the job through the Tokyo [donor] conference in July and, as is our wont, we will squeeze every ounce of value out of him."
When asked by a reporter whether the burden of the job had taken a toll on Crocker's health, Nuland said she would allow the ambassador himself to provide more detail in the future.
"We simply wanted and [Crocker] wanted to make it clear that this should not in any way be seen as a lessening of his personal commitment and our national commitment, obviously, to Afghanistan," she said.
Crocker's departure comes as the United States and its NATO allies prepare to hand over security responsibilities to Afghan forces by mid-2013 and withdraw all combat troops from the country by the end of 2014.
U.S. officials said Crocker made his plans known to Obama during this weekend's NATO summit in Chicago, at which the bloc endorsed plans to wind down the decade-long war.
'Heart In Afghanistan'
Crocker was in Afghanistan in 2002 as charge d'affaires to reopen the U.S. Embassy after the Taliban regime was toppled.
More recently, he had played a key role in contentious negotiations with the government of Afghan President Hamid Karzai that resulted in a strategic partnership agreement between the countries.
The deal, signed on May 1, charts the next decade of relations between the countries and allows the United States to potentially keep troops in Afghanistan after the war ends for training and select counterterrorism missions.
Crocker was known throughout his career for maintaining calm under fire, even as he served in some of the world's most dangerous hotspots.
When a Pakistan-based group allied with the Taliban attacked the U.S. Embassy in Kabul in September 2011, taking over a nearby building and firing rockets and bullets at the compound during a 20-hour siege, Crocker said, "If that's the best they can do, you know, I think it's actually a statement of their weakness."
During his service in Iraq, Crocker oversaw the civilian side of the U.S. surge under former President George W. Bush.
Alongside General David Petraeus, he helped lay the groundwork for eventual U.S. withdrawal from the country.
In 2009 he was awarded the Presidential Medal of Freedom by President Bush.
He became dean of Texas A&M University's George Bush School of Government and Public Service before being called back into duty by Obama.
Crocker's current deputy, James Cunningham, will replace him in the interim before the White House nominates a permanent replacement. Cunningham is considered a leading candidate.
With reporting by AP and Reuters