The new transition planning comes as prospects for last year’s troop increase in Afghanistan and reformulated strategy there remain uncertain. American forces in Afghanistan have tripled under Mr. Obama, and Gen. David H. Petraeus, the commander, has expressed confidence that they are making progress. But the last of the reinforcements arrived only recently, and officials in Washington have said it is too early to say whether the strategy will work.

Any such transition risks declaring Afghan units combat-ready before they really are, and officials emphasized Sunday that any transition would be based on local conditions, not a dictate from Washington, and would be a process, not an event. “This will be ground-up,” one official said.

The American government is already assessing which areas could be safely handed over to Afghan security forces and will be ready to identify them late this year or early next year, officials said. Every few months, more areas will begin the transition, with the last at the end of 2012. Those will almost certainly include the toughest areas, like Khost in the east and Kandahar in the south.

Even after Afghan forces have assumed the lead in a province, some American or NATO forces may remain or be positioned “over the horizon” elsewhere in Afghanistan ready to respond quickly if necessary. By the end of 2014, American and NATO combat forces could be withdrawn if conditions warrant, although tens of thousands very likely will remain for training, mentoring and other assistance, just as 50,000 American troops are still in Iraq.

The plan came amid escalating pressure from President Hamid Karzai of Afghanistan to reduce the visibility of American troops, to halt night raids unless carried out by Afghan soldiers or police officers and to begin withdrawing foreign forces by next year. “The time has come to reduce military operations,” Mr. Karzai told The Washington Post in an interview that stirred renewed concern among American officials on Sunday. “The time has come to reduce the presence of, you know, boots in Afghanistan.”