Washington will have to stand by Afghanistan for years, training and funding its army until Kabul can sign a peace deal with the Taliban, or it will follow Iraq in sliding into chaos, experts warn.

"Iraq shows the big security and political price you can pay down the road for over-reliance on a local ally to maintain security without a US presence," warned Stephen Biddle, senior fellow at the Council on Foreign Relations.

The biggest challenge for the United States will be the complex and painstaking task of building and institutionalizing a national Afghan army in a country that is plagued by competing warlords and has known decades of foreign occupation and civil strife.

On Tuesday, the day-old Afghan government but an end to months of delay by its predecessor and signed a bilateral security deal (BSA) with the United States governing the presence of American forces beyond the end of this year.

A similar deal was inked with NATO, allowing 12,500 foreign troops, including 9,800 Americans, to stay behind in 2015 after the alliance's combat mission ends.

By January 2017 there will be no American forces left, more than a decade after the US jumped into the ongoing Afghanistan conflict in 2001 to oust Taliban militants, blamed for sheltering the Al-Qaeda militants who masterminded the 9/11 attacks.

But experts warn that pulling out all US troops will only leave another vacuum, as it did in Iraq, where US-trained and funded forces crumbled this year in face of a lightning onslaught by Islamic State militants.

"If we stay on the 2017 zero option time line Afghanistan will be another Iraq," said former CIA veteran Bruce Riedel, now an expert with the Brookings Institution.

Setting "an arbitrary deadline" for a complete withdrawal "has undermined the mission recklessly," Riedel insisted.

"The whole region believes we are cutting and running by 2017 which is creating its own momentum."

- Afghan forces fighting back -

Obama abruptly pulled all American troops out of Iraq in late 2011 when the US and Iraqi governments failed to reach a deal governing their future presence.

Many -- Republicans as well as some more impartial observers -- have since chided the administration for leaving, saying it created a security vacuum in a fragile country riven by sectarian politics.

Indeed, at the weekend Obama admitted the United States had underestimated the threat posed by the Islamic State militants, while overestimating how effective Iraqi security forces would be against them.

But US ambassador James Cunningham on Wednesday insisted the world should have faith in the Afghan forces.

"Unlike what happened in Iraq, the Afghan security forces have stood, even as they have been taking very significant casualties over the summer," Cunningham told reporters.

"They have stood and they have fought and, where they had setbacks, they have re-grouped and they have come back and re-established themselves, and I don't see reason for that to change in the coming years and indeed they can only get better."

NATO's new mission -- dubbed Resolute Support -- will focus on supporting Afghan forces as they take on the Taliban, in parallel with US counter-terrorism operations.

The Taliban described the signing of the BSA as "embarrassing and regrettable."

But undeterred, new President Ashraf Ghani called on the Taliban and another militant group, the Hizb-e-Islami, to enter talks.

"Any problems that they have, they should tell us, we will find a solution," Ghani said.

"We ask every villager to call for peace. We ask Muslim scholars to advise the Taliban, and if they don't listen to their advice, they should cut off any relations."

- Building an army -

In order to ensure the long-term viability of the Afghan security forces (ANSF), the US and its allies will have to dig deep to secure funding.

"The way to avoid a debacle like Iraq in Afghanistan is to keep the ANSF funded and fighting long enough to negotiate a settlement with the Taliban," said analyst Biddle.

Congress will not fund the Afghan security forces forever, he predicted.

A negotiated peace "prior to Congressional defunding is the only way to an acceptable outcome - without a settlement, the ANSF will eventually break up when the Congress eventually cuts their funding."

The United States must also work to ensure the Afghan army becomes one of the nation's pillars, much in the way Britain achieved in India, said retired general Paul Eaton, who served in Iraq and is now an analyst with the National Security Network.

Britain's legacy was to shape a force such that a young army conscript "believed in his country and would do audacious things, not for money, but for other soldiers and for the state of India."

"That is a very much political evolution that takes many many years that can't be done overnight."