International troops may remain in Afghanistan beyond the May deadline envisaged by the Taliban's deal with the US, four senior Nato officials said.
The move could increase tensions as the insurgents are demanding complete withdrawal.
"There will be no full withdrawal by allies by April-end," one official said.
"Conditions have not been met," he said. "And with the new US administration, there will be tweaks in the policy. The sense of hasty withdrawal which was prevalent will be addressed and we could see a much more calculated exit strategy."
The Pentagon on Thursday said the Taliban’s refusal to meet commitments to reduce violence in Afghanistan raised questions about whether all US troops will be able to leave by May as required under the peace agreement.
Pentagon chief spokesman John Kirby said the US stands by its commitment for a full troop withdrawal, but the agreement also calls for the Taliban to cut ties with Al Qaeda and reduce violence.
Echoing what senior military and defence leaders asserted in recent months, Mr Kirby said the Taliban has not yet met the requirements set in the peace agreement.
“Without them meeting their commitments to renounce terrorism and to stop the violent attacks against the Afghan National Security Forces, it’s very hard to see a specific way forward for the negotiated settlement,” Mr Kirby said. “But we’re still committed to that.”
The administration of president Donald Trump signed an agreement with the Taliban last year that called for the withdrawal of all foreign troops by May in return for the insurgents honouring security guarantees.
Mr Trump said the accord – which did not include the Afghan government – as the end of two decades of war. He reduced US troops to 2,500 by this month, the fewest since 2001.
Plans on what will happen after April are now being considered and likely to be a top issue at a Nato meeting in February, the Nato sources said.
The positions of the North Atlantic Treaty Organisation are becoming increasingly important after the alliance was sidelined by Mr Trump, diplomats and experts said.
Peace talks between the Afghan government and the Taliban began in September in Doha, but violence has remained high.
"No Nato ally wants to stay in Afghanistan longer than necessary, but we have been clear that our presence remains conditions-based," said Nato spokeswoman Oana Lungescu. "Allies continue to assess the overall situation and to consult on the way forward."
She said about 10,000 troops, including Americans, are in Afghanistan.
Those levels are expected to stay roughly the same until after May, but the plan beyond that is not clear, the Nato source said.
Kabul and some foreign governments and agencies said the Taliban has failed to meet the conditions, which the Taliban denies.
The administration of Joe Biden, who replaced Mr Trump on January 20, launched a review of the peace agreement.
A Pentagon spokesman said the Taliban have not met their commitments, but Washington remained committed to the process and had not decided on future troop levels.
A State Department representative said Mr Biden was committed to bringing a "responsible end to the 'forever wars' … while also protecting Americans from terrorist and other threats".
The Taliban have become increasingly concerned in recent weeks about the possibility that Washington might change aspects of the agreement and keep troops in the country beyond May, two Taliban sources said.
"We conveyed our apprehensions, but they assured us of honouring and acting on the Doha accord. What's going on, on the ground in Afghanistan, is showing something else. And that's why we decided to send our delegations to take our allies into confidence," said a Taliban leader in Doha.
A Taliban delegation this week visited Iran and Russia, and the leader said they were contacting China.
Although informal meetings have been taking place between negotiators in Doha, progress has stalled in recent weeks after an almost one-month break, according to negotiators and diplomats.
Taliban spokesman Zabihullah Mujahid said the insurgents remained committed to the peace process.
"No doubt that if the Doha deal is not implemented there will be consequences, and the blame will be upon that side which does not honour the deal," he said.
"Our expectations are also that Nato will think to end this war and avoid more excuses for prolonging the war in Afghanistan."
Nato and Washington will have a challenge getting the Taliban to agree to an extension beyond May.
If the situation remains unclear, the Taliban may increase attacks, possibly once again on international forces, said Ashley Jackson, co-director of the Centre for the Study of Armed Groups at the British think tank the Overseas Development Institute.
The lack of a resolution "gives voice to spoilers inside the Taliban who never believed the US would leave willingly, and who have pushed for a ratcheting up of attacks even after the US-Taliban deal was agreed", she said.
A meeting of Nato defence ministers in February will be a chance for the newly empowered alliance to determine how the process would be shaped, said one source, a senior European diplomat.
"With the new administration coming in there will be a more cooperative result, Nato countries will have a say."