Defense Secretary Leon Panetta wants to see U.S. troops shift to a training and advisory role sometime in 2013.
Panetta didn't say how many troops might be required to fill that role, but a leaked NATO report is raising concerns about Afghanistan's future without coalition forces.
In 2010, NATO leaders agreed that Afghan forces would take control province by province until they have full responsibility for security in all of Afghanistan by the end of 2014. Until now, it was widely assumed that coalition troops would retain the lead role in military operations until that final handover.
Panetta is in Brussels this week to meet with NATO defense ministers.
The meeting follows a series of attacks by members of Afghan forces on NATO troops or advisers. There have been at least 35 attacks on international troops since 2007 by Afghan soldiers, police or insurgents wearing their uniforms, according to a tally by The Associated Press. The number rose sharply last year to 17, up from six in 2010.
The Associated Press contributed to this report.