Obama may take weeks to decide whether to add more troops, but the idea of pulling out isn't on the table as a way to deal with a war nearing its ninth year, White House press secretary Robert Gibbs said.

"I don't think we have the option to leave. That's quite clear," Gibbs said.

The question of whether to further escalate the conflict after adding 21,000 US troops earlier this year is a major decision facing Obama and senior administration policy advisers this week.

Obama also invited a bipartisan group of congressional leaders to the White House on Tuesday to confer about the war. And Obama will meet twice this week with his top national security advisers.

Divided on Afghanistan, Congress takes up a massive defense spending bill this week even before the president settles on a direction for the war.

At issue is whether US forces should continue to focus on fighting the Taliban and securing the Afghan population, or shift to more narrowly targeting al-Qaida terrorists believed to be hiding in Pakistan with unmanned spy drones and covert operations.

Gates and Clinton said Monday the goal for the war remains to disrupt al-Qaida and prevent it from again threatening the United States, but they added that a reassessment of the means to do that is appropriate. Speaking to CNN during a rare joint interview with Gates, Clinton said a "snap decision" about the next step would be counterproductive. The interview will air Tuesday.

Gates and some other advisers appear to favor a middle path. A hybrid strategy could preserve the essential outline of an Afghan counterinsurgency campaign that McChrystal rebuilt this summer from the disarray of nearly eight years of undermanned combat, while expanding the hunt for al-Qaida next door.

"Speaking for the Department of Defense, once the commander in chief makes his decisions, we will salute and execute those decisions faithfully and to the best of our ability," Gates told the annual meeting of the Association of the US Army.

The top three US military officials overseeing the war in Afghanistan favor continuing the current fight against the Taliban, and have concluded they need tens of thousands more US troops beyond the 68,000 already there.

Officials across the Obama administration have acknowledged that the Taliban is far stronger now than in recent years, as underscored by the US deaths in Nuristan province.

The fighting Saturday marked the biggest loss of US life in a single Afghan battle in more than a year. It also raised questions about why US troops remained in the remote outposts after McChrystal said he planned to close down isolated strongholds and focus on more heavily populated areas as part of his new strategy to focus on protecting Afghan civilians.

Also being considered as part of a potential force increase is the impact on troops who are already stretched thin from fighting in two wars. Army Chief of Staff Gen. George Casey told reporters that he cannot rule out extending the length soldiers are sent to fight - from 12 months to 15 - although "I would hope we don't get there."

Casey also signaled that the year that soldiers are currently guaranteed at home between deployments could be at risk.

"Simple math: The more troops you have deployed, the less time they'll spend at home," Casey said Monday.