But Mr. Crocker’s comments were an explicit acknowledgment that the post-2014 forces may include combat troops, not just the trainers and advisers who had been publicly mentioned before.

His comments came as the administration was engaged in discussions with the Afghan government on arrangements after 2014. At a conference in Bonn, Germany, last week, President Hamid Karzai and other Afghan officials called for political and military support for at least another decade.

Referring to the NATO summit meeting in Lisbon last year at which Western leaders agreed to transfer security responsibility to Afghan forces by 2014, Mr. Crocker said: “There is nothing in the Lisbon declaration on 2014 that precludes an international military presence beyond 2014. That is to be determined by the parties, who could be numerous, not just us, as we get closer to that date.”

In June, President Obama announced that American troop withdrawals would begin the following month, with 10,000 of the roughly 101,000 American troops then in the country to leave by Dec. 31, and an additional 23,000 to follow by the summer of 2012. “Our troops will continue coming home at a steady pace as Afghan security forces move into the lead,” he said. “Our mission will change from combat to support. By 2014, this process of transition will be complete, and the Afghan people will be responsible for their own security.” Of the first 10,000, 4,000 have left, according to a senior NATO official. In most of those cases, personnel who had been scheduled to leave were not replaced, the official said.

“We are on a timeline, as you know,” Mr. Crocker said. “Ten thousand out by the end of the year, that is being met.” With the additional 23,000 by September 2012, he added, “that basically recovers the surge” — the reinforcements Mr. Obama ordered two years ago.