U.S. ambassador to Afghanistan Ryan Crocker is stepping down from his position in the coming months, even as the United States and its NATO allies work their way out of the costly war.

In a statement, the U.S. Embassy in Kabul said Crocker was confirming “with regret” that he will leave as ambassador sometime mid-2012. Crocker left retirement last July to become the U.S. envoy. He had reopened the embassy in the Afghan capital in January 2002.

It was not immediately clear why the veteran diplomat is stepping down.

The announcement comes day after U.S. President Barack Obama concluded a NATO summit in Chicago that reportedly closed with a clear road map for the alliance's role in Afghanistan.

Mr. Obama said NATO members are unified behind a plan to wind down the war in Afghanistan and have the country take command of its security by 2014.

But the president said work remains, including finding a solution to problems with Afghanistan's neighbor Pakistan. Mr. Obama said Pakistan and the United States are making “diligent progress” in talks to reopen Pakistani borders to NATO supply convoys.

“I think ultimately everybody in the alliance — all of ISAF, and most importantly the people of Afghanistan and Pakistan — understand that neither country is going to have the kind of security, stability, and prosperity that it needs unless they can resolve some of these outstanding issues and join in common purpose with the international community in making sure these regions do not harbor extremists.”

Mr. Obama spoke briefly with Pakistani President Asif Ali Zardari on the sidelines of Monday's meetings.

He also used the summit to thank other nations in the region for allowing expanded NATO shipments through their territory after Pakistan closed its borders to the convoys last November. The closing followed U.S. airstrikes that mistakenly killed 24 Pakistani soldiers along the Afghan border.

The incident brought U.S.-Pakistani relations to a new low. But the two sides are now engaged in intense negotiations to finalize a deal to reopen the routes. Pakistan is seeking heavy taxes on future NATO convoys, a condition diplomatic sources say is hindering the talks.

Pakistan's ambassador to the U.S., Sherry Rehman, said Monday there is hope the talks will be “concluded constructively as soon as possible.”

“It's not just a question about the price mechanism. They are looking at a larger framework for building a memorandum of understanding that is transparent.”

About 130,000 foreign troops remain in Afghanistan as part of the U.S.-led NATO coalition.

Funding for Afghan security forces after 2014 could cost $4.1 billion a year, with the United States expected to cover a large portion of the bill.