Ambassador Ryan C. Crocker is leaving his post a year early. It is known at the embassy that he suffers from severe back pain. (Johannes Eisele/Getty Images)

Ryan C. Crocker, the U.S. ambassador to Afghanistan, said Tuesday that he will leave his job this summer, just one year after President Obama called him out of retirement to manage the crucial embassy in Kabul.

In a letter to his embassy staff, Crocker said that he was leaving “with regret” and that “health issues force my departure earlier than I wanted.” He did not provide specifics, but it is widely known at the embassy that Crocker suffers from severe back pain.

The State Department also announced the retirement of Jeffrey D. Feltman, the assistant secretary of state for Near East affairs. Administration officials confirmed that Feltman — the top diplomat to a volatile region including Syria, Yemen and Egypt — will become undersecretary for political affairs at the United Nations, a senior international post.

Departures near the end of a presidential term are not uncommon. Secretary of State Hillary Rodham Clinton has said that she does not plan to serve in a second Obama administration, if there is one.

Crocker’s and Feltman’s departures come at a particularly delicate time. Cameron Munter, the U.S. ambassador to Pakistan, has also said he will leave this summer after less than two years in what is usually a three-year posting.

Crocker, who previously served as ambassador to Iraq, Pakistan, Syria, Kuwait and Lebanon, rejected several administration invitations to return to the Foreign Service from his post as dean of the school of government at Texas A&M University before he accepted a personal appeal from Obama to take over the troubled embassy in Kabul last July.

During his tenure, testy relations between the embassy and the U.S. military command, the embassy and the White House, and the administration and Afghan President Hamid Karzai have calmed considerably, although there have been sporadic clashes.

Crocker attended Monday’s NATO summit in Chicago, where the alliance agreed to turn lead responsibility for Afghan security over to that country’s military forces by the middle of next year, as the coalition prepares to withdraw all of its 132,000 combat troops by the end of 2014.

More important for U.S. diplomats in Afghanistan, the embassy is in charge of making sure that improvements in that nation’s political and economic difficulties coincide with the coalition military departure.

In his letter to the embassy staff, Crocker said he will stay in office until after an international conference, set for July 8 in Tokyo, at which the Afghans will outline their economic and development plans for the next decade and donor nations are expected to pledge support beyond 2014.

“We will squeeze every ounce of value out of him” before he leaves, State Department spokeswoman Victoria Nuland said of Crocker. She said that no replacement had been named but that Deputy Ambassador James B. Cunningham, a former U.S. ambassador to Israel, would “steer the ship in the interim.”

Feltman, who was traveling outside the country and could not be reached for comment, is due to depart May 31. He has spent most of his career in the Middle East, as an ambassador to Lebanon and in numerous other senior policy and embassy posts.

Administration officials, who spoke on the condition of anonymity because there has been no U.N. announcement, said Feltman will replace B. Lynn Pascoe, also a former U.S. diplomat, who has been in the job five years.

As one of the top five U.N. positions, the undersecretary serves as chief political adviser to Secretary General Ban Ki-moon. Feltman will oversee all U.N. non-peacekeeping missions around the world, including Kofi Annan’s mediation effort in Syria and missions in Somalia, Iraq and other hot spots.

Asked who would replace Feltman, Nuland said, “We are working on that, but I don’t have anything to announce.”

Staff writer Colum Lynch at the United Nations contributed to this report.