The Obama administration has signalled that 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 keep a lid on al-Qaeda and to strengthen Afghan forces.

"The US does not have an inherent objective of 'X' number of troops in Afghanistan," said Ben Rhodes, a White House deputy national security adviser. "We have an objective of making sure there is no safe haven for al-Qaeda 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 US now has 66,000 troops in Afghanistan, down from a peak of about 100,000 as recently as 2010. The US 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.