US officials have warned of the potential for catastrophe if Afghan President Hamid Karzai fails to sign a security pact to allow foreign forces to stay in Afghanistan beyond 2014.

Unless a deal is reached to enable a modest US force of perhaps 8,000 to stay in the country, the Taliban might stage a major comeback, al-Qaida might regain safe havens and Afghan forces might find themselves starved of funding, the officials say.

The post-2014 US force envisioned would train and help Afghan soldiers and go after dangerous militants.

But even if the Obama administration abruptly pulls out its entire force of 43,000 a year from now, it would still retain a handful of limited security options.

While US officials have not discussed a possible post-withdrawal scenario in public, Washington might still, even under those circumstances, continue to provide small-scale support to local forces, mount some special forces missions and use drones to counter al-Qaida and help keep the Taliban at bay.

A narrowed security mission would in many ways track a decade-long shift in US strategy, away from the counter-insurgency campaigns of the 2000s toward the Obama administration's preference for low-profile support to local forces combined with targeted operations.

Even so, full withdrawal of the main US force would make it more difficult to prevent al-Qaida militants regrouping along the wild Afghanistan-Pakistan border and to stop the Taliban from solidifying control of their southern Afghan heartland.

"We have a lot of capabilities, but without the (Bilateral Security Agreement), we are very limited," a US defense official said on condition of anonymity, referring to the bilateral pact the US is seeking with Karzai.

For now, US officials remain hopeful - in public at least - that Karzai will drop last-minute demands and sign the pact well before Afghan elections in April. They say they have not begun to plan for a full withdrawal or a possible post-withdrawal mission in earnest.

But General Joseph Dunford, who commands international forces in Afghanistan, told reporters in Kabul recently, "If there's not an answer in December, I expect that we'll begin to do some more detailed planning about some other eventuality besides the (post-2014) mission."

Another US defense official said that to understand what options the US might have in Afghanistan following a full withdrawal, "you can look to places where we are already active in countering terrorism, like Iraq, Libya and Somalia".

Even if all foreign troops withdraw from Afghanistan, the US might still send small numbers of special forces, such as Green Berets, to do limited, short-term training missions at the request of Afghan officials. They might also launch occasional raids against militants, as they have in Libya or Somalia.

In Iraq, following the US military withdrawal in 2011, Washington set up a large security office attached to its embassy in Baghdad to oversee military sales and provide limited support and advice to the Iraqi government.

US special forces have also been invited to return to Iraq to provide counter-terrorism and intelligence support to Iraqi forces, the general who headed that office said last year, according to a report in The New York Times.

The US military is also providing some training and equipment to security forces in Yemen, defense officials have said, as the Obama administration seeks to weaken al-Qaida and other militants in the Arabian Peninsula.

Robert Grenier, a former director of the Central Intelligence Agency's Counterterrorism Center, said that if withdrawal of the main US force from Afghanistan becomes necessary, Washington should consider putting some special forces under CIA authority to train local forces or perform limited counter-terrorism activities.

Reuters