WASHINGTON - US President Barack Obama said on Friday that the NATO forces in Afghanistan will move to a support role "this spring," advancing a shift that was originally set for this summer.

Addressing a joint press conference with visiting Afghan President Hamid Karzai after talks at the White House, Obama took note of the ongoing transition of lead security to the Afghan forces and the military advances made against the al-Qaida and the Taliban.

"Today we agreed that, as Afghan forces take the lead and as President Karzai announces the final phase of the transition, coalition forces will move to a support role this spring," Obama said.

"Starting this spring, our troops will have a different mission: training, advising, assisting Afghan forces," he added. "It will be a historic moment and another step toward full Afghan sovereignty."

As a result, the US military will further reduce its presence in Afghanistan, which stands at some 66,000 now. In all, the NATO- led forces have some 100,000 troops in Afghanistan.

At their summit in Chicago in May last year, NATO leaders agreed to shift to a support role in mid-2013 and withdraw most of their combat troops by the end of 2014.

In terms of the specific pace of the withdrawal, Obama said. " In the coming months, I'll announce the next phase of our drawdown, a responsible drawdown that protects the gains our troops have made."

Karzai was pleased about the new arrangement, saying "I'm very happy to hear from the president, as we also discussed it earlier, that in spring this year, the Afghan forces will be fully responsible for providing security and protection to the Afghan people."

"The international forces, the American forces, will be no longer present in Afghan villages," he said. "The task will be that of the Afghan forces to provide for the Afghan people security and protection."

Right now, the United States and Afghanistan are still negotiating a security agreement on the level of the US military presence in Afghanistan in post-2014 period. The White House, concerned by the potentially huge human and financial costs it could entail, is still weighing the size of the residual forces. The options reportedly range from as low as 3,000 to as many as 15, 000. ?

But Obama stressed at the press conference that any US troops in Afghanistan beyond 2014 must be at the "invitation" of the Afghan government and they must have legal immunity.

"Nowhere do we have any kind of security agreement with a country without immunity for our troops," Obama said.

"From my perspective at least, it will not be possible for us to have any kind of US troops presence post-2014 without assurances that our men and women who are operating there are in some way subject to the jurisdiction of another country," he added.

For his part, Karzai said that the number of US troops staying beyond 2014 is for Washington to decide.

He also noted that with outstanding issues between the United States and Afghanistan being resolved, he can "go to the Afghan people and argue for immunity for US troops in Afghanistan."