Panetta's unannounced trip to Kabul came as Obama moves to wind down the unpopular 11-year-old war, weighing the pace of a troop withdrawal and a future follow-on force after 2014, when the NATO-led mission is due to end.

With Obama poised to make key decisions, Panetta said he wanted to discuss options on troop numbers with the commander of US and NATO forces in Afghanistan, General John Allen.

"There will begin to be a drawdown that will take us toward the end of 2014," Panetta said earlier during a talk with soldiers and airmen at an American air base in Kuwait.

"At that time, the agreement is that we'll have an enduring presence that will continue in Afghanistan.

"The size of that enduring presence is something that the president is going to be considering in these next few weeks to determine exactly what that will be," he said.

There is growing speculation about the size of a post-2014 force, amid signs White House advisers are pushing for a smaller contingent of less than 10,000 while military officers favor a larger presence of up to 15,000 soldiers.

Panetta flew from Kuwait to Kabul for a two day visit. He will hear a progress report on efforts to hand over security to Afghan army and police as well as fledgling negotiations for an accord with the Afghan government on the terms of a future US military role.