"We wouldn't rule out any option," including zero troops, Ben Rhodes, a White House deputy national security adviser, said Tuesday.
"The U.S. does not have an inherent objective of 'X' number of troops in Afghanistan," Rhodes said. "We have an objective of making sure there is no safe haven for al-Qaida in Afghanistan and making sure that the Afghan government has a security force that is sufficient to ensure the stability of the Afghan government."
The U.S. now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010. The U.S. and its NATO allies agreed in November 2010 that they would withdraw all their combat troops by the end of 2014, but they have yet to decide what future missions will be necessary and how many troops they would require.
Those issues are at the top of the agenda in talks this week as Afghan President Hamid Karzai meets with President Barack Obama on Friday and Defense Secretary Leon Panetta and Secretary of State Hillary Rodham Clinton on Thursday.
At stake is the risk of Afghanistan's collapse and a return to the chaos of the 1990s that enabled the Taliban to seize power and provide a haven for Osama bin Laden's al-Qaida network. Fewer than 100 al-Qaida fighters are believed to remain in Afghanistan, although a larger number are just across the border in Pakistani sanctuaries.