"We will know by the end of December 2010 whether or not the approach that General McChrystal (the top US commander in Afghanistan) has discussed in terms of securing population centres is meeting its objectives," Obama said.
"And if the approach that's been recommended doesn't work then, yes, we're gonna be changing approaches," he said.
Early this month, Obama announced sending 30,000 more troops to Afghanistan into the eight-year-old war, which he said was one of the most difficult decisions of his presidency.
"What I have said is that we then start transitioning into a draw-down phase. The pace of that drawdown, how many US troops are coming out, how quickly, what the slope is of that drawdown will be determined by conditions on the ground," Obama said in an interview to CBS News.
"We are gonna be making consistent assessments to make sure that as we are standing up Afghan troops, that we are replacing US troops or ally troops, and we're not gonna do it in a precipitous way that in any way endangers our troops or endangers the progress that we've made," he said.
"But we will have signalled the fact that increasingly we are turning over responsibility for Afghanistan to the Afghans," Obama said.
As commander-in-chief, Obama said he reserves the option to do what he think would be the best for the American people at that point in time.
"I think the mistake that was made in the early 1990s after the Soviets finally pulled out of Afghanistan was that we just completely lost interest. We didn't think about their development needs... And that's not what we're talking about here. We are gonna be there for the long haul.
"The question is, are we going to have an enormous troops presence in that region for the long haul? I actually think that would be counterproductive," Obama said.
When asked why he has set a deadline for troop withdrawal, he said "open-ended commitment" to Afghanistan is neither in the interest of the US nor in that of the Afghans.
Describing the borders between Pakistan and Afghanistan as the "epicentre of terrorism" where top Al Qaeda leadership lives, Obama said the US presence there has been helpful in pinning them in and restricting their movement and their ability to engineer high profile attacks against the West.
"Although we recognise a boundary between Afghanistan and Pakistan, Al Qaeda and militants don't recognise that boundary. So, what you have here between the borders of Afghanistan and Pakistan is the epicentre of violent extremism directed against West and directed against the US," he said.
"This is this is the heart of it. This is where bin Laden is. This is where its allies are. It's from here that you see attacks launched not just against the United States, but against London, against Bali, against a whole host of countries," Obama said.
