"Starting in July 2011, we'll begin to draw down those additional troops, but we're not going to suddenly leave, turn off the lights, and go home on that date."
Relations between Iran and the US have been strained over Tehran's controversial nuclear activities.
The US fears Iran's uranium enrichment programme is a cover to build atomic weapons, a charge Iranian officials deny.
Mr Obama defended the international sanctions regime against Iran, including new measures adopted by the UN earlier this year.
"This is not a matter of us choosing to impose punishment on the Iranians," he told the BBC. "This is a matter of the Iranian government ultimately betraying the interests of its own people by isolating it further."
And he pointed out that countries such as Russia and China had also backed the UN sanctions.
"Most of these sanctions are targeted at the regime, at its military, and we think that over time, hopefully, there's enough reflection within the Iranian government, that they say to themselves, you know, 'This is not the best course for our people. This is not the best course for Iran.'"
Meanwhile, an Iranian official may meet a representative of the major powers next month to discuss the nuclear standoff, Mr Ahmadinejad said on Friday at the UN.
Iran was always ready for a dialogue based on respect and justice, the Iranian president added.
British diplomatic sources gave a cautious welcome to the prospect of negotiations, but said the Iranians "seem to be talking about talks about talks".