The US will seek to wind down combat operations in Afghanistan during 2013, more than a year before a deadline for withdrawal, the defence secretary says.
Speaking while travelling to a Nato summit, Leon Panetta said the US hoped to switch to a role training and supporting Afghan forces.
It is the most detailed public indication of when US troops might pull back from America's longest war.
Some 68,000 troops are due to remain in Afghanistan after the end of 2012.
There are currently some 99,000 US troops in the country, with 22,000 scheduled to return home during this year.
Until now, though, there had been little information how the Pentagon planned to manage the main bulk of the drawdown, committing only to a full withdrawal of troops by the end of 2014.
Leaked Nato report
"Hopefully by mid- to the latter part of 2013 we'll be able to make a transition from a combat role to a training, advice and assist role," Mr Panetta said en route to Brussels, site of the Nato summit.
Senior military figures have previously stated that the US mission will change from combat operations to training and assistance, but correspondents say Afghans taking the lead is unlikely to mean that US forces will cease patrols.
"It's still a pretty robust role that we'll be engaged in. It's not going to be a kind of formal combat role that we are now," he said. "That doesn't mean that we're not going to be combat-ready. We will be because we always have to be in order to defend ourselves."
The announcement comes shortly after news of a leaked secret Nato report, which said that the Afghan Taliban are helped by Pakistani security services. The report also details widespread collaboration between the insurgents and Afghan police and military.
Pakistan's foreign minister immediately dismissed any allegations that militants might get Pakistani support, but on Thursday she said that Islamabad would do whatever is expected to achieve a lasting peace.
She said that an effective peace process appeared to be a distant prospect but said it should be "Afghan-led, Afghan-owned, Afghan-driven".
Reluctance to commit
President Barack Obama ordered a "surge" in US troops in Afghanistan in 2009 in an effort to seize and retain Taliban-controlled areas.
The US has fought in Afghanistan since shortly after the 9/11 attacks of 2001, seeking to oust the Taliban for their role in harbouring al-Qaeda.
Plans for peace talks between the US and the Taliban are being made. The insurgents so far insisted they will only talk to the US and other allies of Kabul, not the Afghan government of Hamid Karzai.
Amid concern in Nato states and a reluctance by some in Europe to commit more troops, Nato agreed at a Lisbon summit in November 2010 to hand over security duties to Afghan forces by the end of 2014.
Mr Panetta said the US would maintain an "enduring presence" in Afghanistan after its combat troops had come home, and said the 2014 deadline was a pan-Nato ambition that all member states were aiming to meet.
Hours before Mr Panetta detailed plans to hand over combat responsibility to Afghan forces, the Pentagon revealed evidence that members of the country's developing army and security forces had carried out a growing number of "insider" attacks on US troops.
Attacks had risen sharply in the past two years, the Pentagon told a congressional committee. Of 42 attacks since 2007, 75% took place in the last two years.
Most attackers were acting alone, not carrying out insurgent orders, although some attackers were insurgents disguised as soldiers.
Lawmakers said the screening process for Afghan forces was "tragically weak" and called for more stringent vetting.