The Pentagon has recommended that thousands of troops may be needed to stay in that country to back up Afghan forces and contain al-Qaeda.
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.
Administration officials in recent days have said they are considering a range of options for a residual U.S. troop presence of as few as 3,000 and as many as 15,000, with the number linked to a specific set of military-related missions like hunting down terrorists.
His statement could be interpreted as part of an administration negotiating strategy. On Friday Karzai is scheduled to meet Obama at the White House to discuss ways of framing an enduring partnership beyond 2014.
The two are at odds on numerous issues, including a U.S. demand that any American troops who would remain in Afghanistan after the combat mission ends be granted immunity from prosecution under Afghan law. Karzai has resisted, while emphasizing his need for large-scale U.S. support to maintain an effective security force after 2014.
The Associated Press contributed to this report.