WASHINGTON  Six months after President Obama decided to send more forces to Afghanistan, the halting progress in the war has crystallized longstanding tensions within the government over the viability of his plan to turn around the country and begin pulling out by July 2011.

Within the administration, the troubles in clearing out the Taliban from a second-tier region and the elusive loyalties of the Afghan president have prompted anxious discussions about whether the policy can work on the timetable the president has set. Even before the recent setbacks, the military was highly skeptical of setting a date to start withdrawing, but Mr. Obama insisted on it as a way to bring to conclusion a war now in its ninth year.

For now, the White House has decided to wait until a review, already scheduled for December, to assess whether the target date can still work. But officials are emphasizing that the July 2011 withdrawal start will be based on conditions in the country, and that the president has yet to decide how quickly troops will be pulled out.

Even if some troops do begin coming home then, the officials said that it may be a small number at first. Given that he has tripled the overall force since taking office, Mr. Obama could still end his term with more forces in Afghanistan than when he began it.