The Pentagon is developing plans to withdraw up to half of the 14,000 American troops serving in Afghanistan, US officials said.
One official said the troops could be out by summer, but no final decision has been made.
President Donald Trump has long pushed to pull troops out of Afghanistan, considering the war a lost cause.
But earlier this year, he was persuaded by Defence Secretary Jim Mattis and others military leaders to keep troops on the ground to pressure the Taliban and battle a stubborn Islamic State insurgency.
Officials said the latest White House push for withdrawal was another key factor in Mr Mattis’s decision to resign on Thursday.
US troops stormed into Afghanistan in November 2001 in an invasion triggered by the September 11 attacks.
Since then, America has lost more than 2,400 soldiers and spent more than 900 billion dollars (£711 billion) in its longest war.
Three US presidents have pledged to bring peace to Afghanistan, either by adding or withdrawing troops, by engaging the Taliban or shunning them, and by struggling to combat widespread corruption in the government.
The US and Nato formally concluded their combat mission in 2014, but American and allied troops remain, conducting strikes on the Islamic State group and the Taliban and working to train and build the Afghan military.
Taliban insurgents, however, control nearly half of Afghanistan and are more powerful than at any time since a 2001 US-led invasion. They carry out near-daily attacks, mainly targeting security forces and government officials.
In recent months, however, there has been a renewed effort to make progress on peace talks with the Taliban.
Officials now worry that any move to withdraw US troops this year could dampen those prospects and simply encourage the Taliban to wait it out until they can take advantage of the gaps when the forces leave.
A spokesman for President Ashraf Ghani said such a withdrawal would have little impact on the fighting capacity of the Afghan National Security Forces.
Haroon Chakansuri said Afghanistan’s military has been in charge of the country’s security since 2014 when more than 100,000 Nato troops withdrew.
Since then, US forces have provided training and advice, assisting in military operations only when requested by Afghan troops.