President Barack Obama's advisers are debating a shift in the military's mission in Afghanistan that would focus on advising Afghan forces while scaling back combat operations much earlier than planned, officials say.

The potential change raises the possibility of a faster-paced drawdown of US troops, but administration officials insist it is too early to speculate on an option that is still tentative.

"It shouldn't surprise anybody that we're thinking ahead about what may happen but it's very preliminary and there are no particular options," a US defense official told AFP.

The Wall Street Journal first broke the story, reporting Thursday that top officials have discussed revising the mission and that the move could possibly be approved as soon as May when NATO leaders meet in Chicago.

A debate on strategy and troop levels in the unpopular war comes as Obama faces a difficult re-election campaign next year and mounting pressure to find savings in the federal budget.

The Pentagon did not deny the report and said only that the administration's policy on the 10-year-old war had not changed.

"There are no formal plans that have been presented from this department to the White House or from this government to the government of Afghanistan on a potential change of mission," press secretary George Little told reporters.

At the last NATO summit in Lisbon last year, alliance members approved a decision to transfer security duties to Afghan forces for the whole of the country by the end of 2014, paving the way for the exit of NATO-led troops.

The possible shift is reminiscent of the administration's approach to Iraq, where a new "advise and assist" mission -- dubbed Operation New Dawn -- was announced in 2010 to underline an emphasis on handing security over to Iraqi forces.

With 97,000 US troops and 45,000 allied soldiers, the NATO-led force has concentrated on rolling back Taliban insurgents in towns and cities while training Afghan army and police.

US commanders have tended to push for more time for combat operations and for delaying withdrawals of troops as much as possible, but some inside and outside the Pentagon are arguing for handing over to Afghan forces sooner while more NATO boots are still on the ground.

"It's better to do it when you've got 80,000 troops on the ground than when you've got 40,000 troops on the ground and you're looking at the exits," said Andrew Exum, a fellow at the Center for a New American Security who has called for changing the mission.

"You've got to make this mental leap away from -- 'I'm going to do this myself' to 'I'm going to enable our Afghan partners to do it,'" said Exum, a retired US Army officer who served in Afghanistan.

An earlier handover would allow a larger allied force to back up Afghan forces when they falter, he said.

The approach also does not necessarily mean an accelerated drawdown of troops but poses the risk that Afghans or other regional players could perceive the change as a retreat.

"By changing the mission, does it look like the US is running for the exits or does it look like the US is making a logical transition?" asked Exum.

The revised mission would likely not affect the US military's manhunts carried out by special operations forces, which target militants linked to Al-Qaeda, Exum and other analysts said.

But the change represents a clear break from the military's experience in Iraq, where "we defeated the insurgency and then turned the country over to the Iraqis themselves," Exum said.

"In Afghanistan, we are going to turn over the country to the Afghans before the insurgency is fully defeated."

Obama's political opponents were quick to question the possible change in mission.

Referring to both the Iraq withdrawal and the potential mission change in Afghanistan, Republican US House Speaker John Boehner said "we should be very careful about losing the gains that have been made because of someone wanting to be more expedient."

The outcome of the war in Afghanistan will hinge in part on the competence and cohesion of Afghanistan's security forces, which have gotten mixed reviews.

NATO has overseen a dramatic expansion of the security forces with the army set to grow to 195,000 and the police to 157,000 by October 2012.

But out of 173 Afghan battalions, only one is designated by the Pentagon as able to operate independently from the US-led coalition.