The U.S. ambassador to Afghanistan, Ryan Crocker, has announced he is stepping down from his post within a few months for health reasons.

U.S. State Department spokeswoman Victoria Nuland said in a statement Tuesday that Ambassador Ryan Crocker will leave his post after international donor conferences for Afghanistan are held in Tokyo and Kabul.

Nuland said Crocker's tenure has been marked by enormous achievements. She listed as highlights the successful negotiation of a strategic partnership agreement with Afghanistan, and the recently-concluded NATO summit in Chicago, which covered Afghanistan and other issues.

It is not immediately clear who will replace Crocker. But U.S. officials said the likely candidate is James Cunningham, deputy ambassador to Kabul.

The announcement came a day after U.S. President Barack Obama concluded the two-day NATO summit, during which alliance members united behind a plan to wind down the war in Afghanistan and have the country take command of its security by 2014.

Mr. Obama said work remains, including finding a solution to problems with Afghanistan's neighbor, Pakistan. He said one of the problems is the reopening of Pakistani borders to NATO supply convoys, which Pakistan closed after a NATO airstrike last November 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.

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

Pakistan's ambassador to the United States, 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.”