The Obama administration says it might leave no troops in Afghanistan after December 2014, an option that defies the Pentagon's view that thousands of troops may be needed to contain al-Qaida and to strengthen Afghan forces.
A White House deputy national security adviser, Ben Rhodes, said Tuesday that President Barack Obama "wouldn't rule out any option," including zero troops.
Obama will discuss the matter Friday with Afghan President Hamid Karzai, although Rhodes said there is little prospect of the leaders announcing an agreement.
The administration is considering other options, ranging from about 3,000 troops to about 15,000.
The U.S. now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010.