WASHINGTON — The United States’ top representative to the Syrian opposition, Robert S. Ford, is planning to retire from the State Department and is expected to leave his post soon, department officials said on Tuesday.

The officials provided no information on his successor.

As the American ambassador in Damascus, Mr. Ford was active in challenging President Bashar al-Assad’s crackdown on the emerging political opposition in Syria.

In September 2011, a pro-government mob tried to break down the door when Mr. Ford went to a meeting with Hassan Abdel-Azim, an opposition figure in Damascus. An American Embassy security detail was sent to the scene, but two of the vehicles were attacked with iron bars and concrete blocks and destroyed. After four hours, the Syrian police finally arrived, and Mr. Ford was able to return to the embassy.

The next month, Mr. Ford was recalled to the United States because of threats to his safety.

He returned to Damascus that December, but in February 2012, as the security situation in Syria sharply deteriorated, the American Embassy was closed. He later became the chief American envoy to the moderate Syrian opposition and played a central role during the first round of the peace talks in Geneva this month.