The United States signed a landmark deal with the Taliban on Saturday, laying out a timetable for a full troop withdrawal from Afghanistan within 14 months as it seeks an exit from its longest-ever war.

The agreement is expected to lead to a dialogue between the Taliban and the Kabul government that, if successful, could ultimately see an end to the grinding 18-year conflict.

Taliban fighter-turned-dealmaker Mullah Baradar signed the accord alongside Washington's chief negotiator Zalmay Khalilzad, at a gilded desk in a conference room in a luxury Doha hotel.

The pair then shook hands, as people in the room shouted "Allahu Akbar" (God is greatest).

US Secretary of State Mike Pompeo looked on as the two inked the deal, after urging the insurgents to "keep your promises to cut ties with Al-Qaeda". 

President Donald Trump urged the Afghan people to embrace the chance for a new future, saying the deal held out the possibility of ending the 18-year conflict.

"If the Taliban and the government of Afghanistan live up to these commitments, we will have a powerful path forward to end the war in Afghanistan and bring our troops home," he said in a message.

Pompeo warned the Taliban to 'keep your promises to cut ties with Al-Qaeda' and said US 'will not hesitate to nullify' the agreement if the Taliban break the deal.

The agreement is expected to lead to a dialogue between the Kabul government and the Taliban that, if successful, could ultimately see the Afghan war wind down.

But the position of the Afghan government, which has been excluded from direct US-Taliban talks, remains unclear and the country is gripped by a fresh political crisis amid contested election results. 

The United States and its allies will withdraw all their forces from Afghanistan within 14 months if the Taliban abide by the Doha agreement, Washington and Kabul said in a joint statement.

After an initial reduction of troops to 8,600 within 135 days of Saturday's signing, the US and its partners "will complete the withdrawal of their remaining forces from Afghanistan within 14 months... and will withdraw all their forces from remaining bases", the declaration stated. 

The US and the Taliban agreed to swap thousands of prisoners in a "confidence building measure."  

"Up to 5,000 prisoners of the (Taliban)... and 1,000 prisoners of the other side (Afghan forces) will be released by March 10," the deal says.

Dialogue between the Kabul government and the Taliban is due to begin by that date.

The Doha accord was drafted over a tempestuous year of dialogue marked by the abrupt cancellation of the effort by Trump in September.

The signing comes after a week-long, partial truce that has mostly held across Afghanistan, aimed at building confidence between the warring parties and showing the Taliban can control their forces.

NATO Secretary General Jens Stoltenberg heralded the agreement as a "first step to lasting peace". 

"The way to peace is long and hard. We have to be prepared for setbacks, spoilers, there is no easy way to peace but this is an important first step," the Norwegian former prime minister told reporters in Kabul.

'Happy and celebrating' 

Since the US-led invasion that ousted the Taliban after the September 11, 2001 attacks, America has spent more than $1 trillion in fighting and rebuilding in Afghanistan.

About 2,400 US soldiers have been killed, along with unknown tens of thousands of Afghan troops, Taliban fighters and Afghan civilians.

The insurgents said they had halted all hostilities Saturday in honour of the agreement.

"Since the deal is being signed today, and our people are happy and celebrating it, we have halted all our military operations across the country," Taliban spokesman Zabihullah Mujahid told AFP.

As many as 30 nations are expected to be represented at Saturday's signing in the Qatari capital. 

While Kabul will not be represented at the Doha ceremony, set for 1245 GMT, it will send a six-person taskforce to the Qatari capital to make initial contact with the Taliban political office, established in 2013.

Blood and treasure

Any insurgent pledge to guarantee Afghanistan is never again used by jihadist movements such as Al-Qaeda and the Islamic State group to plot attacks abroad will be key to the deal's viability.

The Taliban's sheltering of Al-Qaeda was the main reason for the US invasion following the 9/11 attacks.   

The group, which had risen to power in the 1990s in the chaos of civil war, suffered a swift defeat at the hands of the US and its allies. They retreated before re-emerging to lead a deadly insurgency against the new government in Kabul.

After the NATO combat mission ended in December 2014, the bulk of Western forces withdrew from the country, leaving it in an increasingly precarious position.

While Afghans are eager to see an end to the violence, experts say any prospective peace will depend on the outcome of talks between the Taliban and the Kabul government.

But with President Ashraf Ghani and rival Abdullah Abdullah at loggerheads over contested election results, few expect the pair to present a united front, unlike the Taliban, who would then be in a position to take the upper hand in negotiations.