Exclusive: U.S. envoy to Afghanistan to step down shortly CHICAGO (Reuters) - Veteran U.S. diplomat Ryan Crocker is expected to step down soon from his post as President Barack Obama's envoy to Afghanistan, departing the U.S. mission in Kabul as the United States negotiates a host of challenges on its course out of the long, costly war. The Obama administration is considering Deputy Ambassador James Cunningham to replace Crocker when he leaves the post as early as this month, sources familiar with the matter said. It was not immediately clear why the widely respected diplomat was leaving. Crocker, who came out of retirement to become ambassador in Kabul in July 2011, has led diplomatic efforts in Afghanistan at a time when the White House has focused on reducing the U.S. presence there - and on doing so without letting the country slide back into civil war. Crocker was one of the high-level officials discussing Afghanistan's future at a NATO summit that concluded on Monday in Obama's home town of Chicago. There, Western leaders refined their plans to pull most of their troops out of Afghanistan by the end of 2014 and put local forces in charge. At the core of Crocker's job has been maintaining a difficult, and at times openly hostile, U.S. relationship with Afghan President Hamid Karzai, and overseeing civilian efforts to combat corruption and poverty in Afghanistan. Crocker has anchored the U.S. diplomatic presence at a time when Western sites - and even the American embassy building itself - have been the target by Taliban militants. Crocker has also urged the Afghan government to make reforms that could help it secure billions of dollars in future outside aid. The appointment of a diplomat of Crocker's stature in 2011 was seen as another sign of Obama's desire to turn around a campaign that was received insufficient attention and resources for years - and of his determination to steer the U.S. mission there toward a conclusion. News of Crocker's departure comes only weeks after Cameron Munter, the U.S. ambassador in Islamabad, informed his staff he would be leaving his post in coming months. In yet another diplomatic departure as Obama's current term enters its final stretch, Reuters reported that Secretary of State Hillary Clinton's top diplomat for the Middle East, Jeffrey Feltman, will soon leave for a top post at the United Nations. Crocker earned respect in his earlier role as U.S. ambassador to Iraq from 2007-2009, when a surge in U.S. forces was credited with helping to calm a country convulsed by sectarian violence. The post in Baghdad when suicide bombings and mortar attacks were a daily occurrence appeared to tax Crocker's health. He has also served as ambassador to Pakistan, Syria, Kuwait and Lebanon. As a young diplomat, he worked at the U.S. embassy in Beirut when it was bombed in 1982 and when an American Marine barracks was attacked the following year. Crocker retired from the government in April 2009, becoming dean of the Bush School of Government and Public Service at Texas A&M University. Cunningham, who would need to be nominated and confirmed by the Senate if he were to become the permanent U.S. envoy, was ambassador to Israel before he came to Kabul last year. Reporting By Missy Ryan; editing by Warren Strobel and Christopher Wilson