President Barack Obama Friday said NATO troops would speed up a transfer of lead security responsibility to Afghan forces this spring, in a sign the pace of US troop withdrawals could quicken.

After meeting President Hamid Karzai to chart Afghanistan's future, Obama insisted the United States had achieved its target there of "decapitating" Al-Qaeda, despite falling short of heady early goals for Afghan reconstruction.

The two leaders met at a crucial moment in the final chapter of a long, bloody war, and as Obama balances the future shape of Afghanistan with US combat fatigue and a desire to spend America's dwindling resources at home.

"Starting this spring, our troops will have a different mission -- training, advising, assisting Afghan forces. It will be a historic moment and another step toward full Afghan sovereignty," Obama told a joint news conference.

"Because of the progress that has been made in terms of Afghan security forces, their capacity to take the lead, we are able to meet (transition) goals and accelerate them somewhat," Obama said.

NATO plans had previously called for foreign forces to transfer lead fighting duties against the Taliban by the middle of the this year. Obama was careful to stress however that US troops will still fight alongside Afghans.

Karzai added that from the spring, "the Afghan forces will be fully responsible for providing security and protection to the Afghan people.

"International forces, the American forces, will be no longer present in the villages ... it will be the task of the Afghan forces to provide for the Afghan people in security and protection."

Obama, planning the withdrawal of most of the 66,000 US troops left in Afghanistan, said that after 2014, American forces would have a "very limited" mission in training Afghan forces and preventing a return of Al-Qaeda.

But he warned that Karzai, with whom he has had a somewhat testy relationship, would have to accept a security agreement, still under discussion, granting legal immunity to US troops who remain behind.

"It will not be possible for us to have any kind of US troop presence post-2014 without assurances that our men and women who are operating there are (not) in some way subject to the jurisdiction of another country."

Karzai announced progress on another sticking point between the sides, saying that the leaders had agreed to a complete return of detention centers and detainees to Afghan control, starting soon after he returns home.

But he would not be drawn on the size of the foreign troop garrison he believes is necessary to support Afghan forces and to ensure that Afghanistan does not slip back into violence and dislocation.

The White House has ordered the Pentagon to come up with plans for a smaller future Afghan presence than the generals expected, perhaps numbering 3,000, 6,000 or 9,000 US troops.

Obama's domestic political opponents, however, charge that the president is in a rush for the exit and warn that a minimal force could squander gains hard won in a war that has killed more than 3,000 coalition troops.

The White House even suggested this week that Obama would not even rule out the possibility of leaving zero American boots on the ground.

This has compounded Afghan fears that the country could be abandoned again by the international community -- as it was after the end of the Soviet occupation in 1989.

The power vacuum led to the rise of the Taliban, and a safe haven for Al-Qaeda to plot the September 11 attacks, which drew the United States into an Afghan war in 2001.

Obama said Friday that despite the huge, human and financial cost of the 12-year-war, it was important to recognize that it had been waged in response to those attacks and had achieved its central goals.

"There is no doubt that the possibility of peace and prosperity in Afghanistan today is higher than before we went in," he said.

"Have we achieved everything that some might have imagined us achieving in the best of scenarios? Probably not. You know, there is a human enterprise and you fall short of the ideal."

Obama and Karzai met for two hours before sitting down together for lunch and holding their joint press conference.

Karzai gave an undertaking that he would stand down at the end of his second term, after elections in 2014, following some concerns that he could try to cling onto power, or influence his eventual successor behind the scenes.

"The greatest of my achievements eventually, seen by the Afghan people, will be a proper, well-organized, interference-free election in which the Afghan people can elect their next president," he said.

"And certainly I will be a retired president, and very happily a retired president," he vowed, in response to a question about the 2014 vote at a joint White House press conference with US President Barack Obama.

Karzai was elected in 2004 and re-elected in 2009, in two votes marred by allegations of widespread electoral fraud and held against the backdrop of an ongoing war between NATO-backed government forces and Taliban guerrillas.