KABUL: A key decision on how many US troops will stay on in Afghanistan after 2014 could be made next week at talks between President Barack Obama and Afghan leader Hamid Karzai in Washington, officials say.

With NATO's combat mission due to end next year, the number of US troops left behind is seen as crucial to the chances of thwarting Al-Qaeda militants and heading off the threat of civil war waged by a resurgent Taliban.

General John Allen, the US commander in Afghanistan, has submitted three options of leaving either 6,000, 10,000 or 20,000 troops in the country, The New York Times has reported, citing defence officials.

Those troops would launch strikes against militants and continue training the Afghan army and police, who will be responsible for national security more than a decade after a US-led alliance ousted the Taliban regime in 2001.

US Defense Secretary Leon Panetta has said the slimmed-down force would focus on preventing Al-Qaeda, which was sheltered by the 1996-2001 Taliban government, from regaining a foothold in the war-shattered nation.

"We hope to arrive at an agreement we can announce during the visit," a US official who spoke on condition of anonymity told AFP, adding that Obama would reassure Afghans that the United States would stand behind its ally.

The number of foreign troops battling the Taliban-led insurgency has fallen to 100,000 from about 150,000. There are currently 66,000 US troops, down from a maximum of about 100,000.

Pentagon spokesman George Little confirmed that the post-2014 force would top the agenda in talks with Karzai.

"General Allen has passed his recommendations to the secretary (Panetta) and we expect within a relatively short period of time to arrive at a final conclusion," Little told reporters this week.

Karzai has expressed support for keeping US troops in Afghanistan but sensitive details -- including immunity for US soldiers and the transfer of detainees into Afghan custody -- are still under negotiation.

His relationship with Washington has been troubled in recent years and there are growing fears within Afghanistan that the country, heavily dependent on international aid, could be abandoned after 2014.

Karzai's deputy spokesman Siamak Herawi said that further progress on a final security agreement was expected during Monday's talks.

"There still persist a few differences of opinion that we hope will be resolved but there is high optimism," Herawi said, declining to give further details.

Karzai has suggested his government is ready to agree to demands for legal protection of US troops after 2014.

Washington scrapped plans for a "residual force" to remain in Iraq after Baghdad refused to grant US soldiers immunity from prosecution.

Analysts say Karzai will seek increased support to boost the Afghan security forces, which many believe will be unable to contain the country's warring factions without US assistance.

"Afghans remember being globally isolated (in the 1990s) and nobody wants to be left alone again, so that is the priority for Karzai," said Kabul-based analyst Omar Sharifi of the American Institute of Afghanistan Studies.

"He is in a transition phase himself with his term in office ending next year and the personal charm he used to rely on in Washington faded long ago.

"It is not just about the number of US troops that remain, but what their role is. Will they go after hideouts of militant groups such as the Haqqani network?"

Obama last visited Kabul in May -- one year after US commandos killed Al-Qaeda chief Osama bin Laden -- when he signed a pact on future relations and declared that the "time of war" was ending in Afghanistan.

The conflict has become increasingly unpopular in the United States, but some US lawmakers have accused Obama of pushing for a hasty exit.

Preliminary talks between the United States and the Taliban in Qatar failed last year over the proposed release of five Guantanamo Bay detainees in return for US soldier Bowe Bergdahl, who has been held hostage since 2009.