US Defense Secretary Leon Panetta conferred with commanders in Afghanistan on Wednesday on how many American troops should remain in the country after most combat forces withdraw in 2014.

Panetta's unannounced visit comes as President Barack Obama moves to wind down the unpopular 11-year war, weighing the pace of a troop withdrawal and a future follow-on force after 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 during a talk with soldiers and airmen at a US 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."

US officials say the administration is looking at a range of scenarios for the post-2014 force. Some White House advisers are pushing for less than 10,000 while senior military officers prefer a larger presence of up to 15,000 soldiers.

The Los Angeles Times reported the Obama administration was considering no more than 9,000 troops to remain after 2014, under a plan that would rely on Bagram airfield while turning over large bases in the south and east to the Afghan government.

The United States has roughly 66,000 troops on the ground, down from a peak of about 100,000 in 2011.

Panetta, who flew from Kuwait for his two-day visit, held talks for more than an hour with Allen and the US ambassador to Kabul, James Cunningham, before dining with a larger group of US and alliance generals.

As reporters looked on at the start of the dinner, Panetta told the top brass he hoped his visit would help "tee up" White House decisions on future troop levels.

In the latest sign of NATO's planned exit, US-led forces are scaling back joint operations with Afghan forces and instead are focusing on air power and logistical support, a senior US general told reporters travelling with Panetta.

While the coalition once stressed "shoulder-to-shoulder" offensives with Afghan troops, the priority now is to have Afghans carrying out operations on their own with Western forces offering back-up, said Major General Larry Nicholson, deputy chief of staff of operations for the NATO force.

"We will push them as far as we can to be self-sufficient, to take care of their own problems," he said.

Nicholson said Afghan forces will be ready to take over after NATO departs, despite their shortcomings.

"Once you get to 2015, it will be imperfect, it will be flawed, it will have warts. But it's going to work," he said.

During what may be his last visit to Afghanistan as Pentagon chief, Panetta was expected to hear an update on negotiations with the Kabul government on the terms of a future US military role.

Any post-2014 force would have to be agreed by Kabul and US officials want legal protections for American and other foreign troops, similar to agreements in other countries that host US soldiers.

Washington recalled all its troops from Iraq after Baghdad refused to grant US soldiers immunity from prosecution.

Afghan President Hamid Karzai, who is due to hold talks with Panetta on Thursday, warned last week that his government might suspend negotiations on the future US force unless Washington hands over custody of all detainees held at Bagram prison.

In his talks with Allen, Panetta did not plan to discuss a sex scandal that has ensnared the outgoing commander, officials said.

The Pentagon inspector general is investigating potentially "inappropriate" email exchanges between Allen and a Florida woman, Jill Kelley, who is linked to the scandal that forced the resignation last month of CIA director David Petraeus, the former four-star general.

Allen has insisted on his innocence but his nomination to be NATO's next supreme allied commander has been put on hold pending the outcome of the probe.

The next commander of NATO's US-led force, General Joseph Dunford, is set to take over his post in the new year.