US President Barack Obama has described as "hateful" and "offensive" the claim by Iran's President Mahmoud Ahmadinejad that most people believe the US government was behind the 9/11 attacks.
Mr Obama was speaking exclusively to BBC Persian television, which broadcasts to Iran and Afghanistan.
Mr Ahmadinejad's speech at the UN General Assembly on Thursday triggered a walkout.
He later defended his remarks and called for an inquiry into the attacks.
"I did not pass judgment, but don't you feel that the time has come to have a fact-finding committee?" Mr Ahmadinejad told reporters on Friday.
"The fact-finding mission can shed light on who the perpetrators were, who is al-Qaeda... where does it exist? Who was it backed by and supported? All these should come to light."
In his speech at the UN, the Iranian leader suggested the US government could have "orchestrated the attack to reverse the declining American economy, and its grips on the Middle East, in order to save the Zionist regime". Mr Ahmadinejad usually refers to Israel as the "Zionist regime".
Mr Obama said it was inexcusable to make such remarks in New York itself, where most of the victims of 9/11 died.
But despite his condemnation, the US president reaffirmed America's commitment to reach out to the people of Iran, who he said had a very different response to 9/11.
"There were candlelight vigils and I think a natural sense of shared humanity and sympathy was expressed within Iran," Mr Obama told the BBC. "It just shows once again the difference between how the Iranian leadership and this regime operates and how I think the vast majority of the Iranian people, who are respectful and thoughtful, think about these issues."
This was in stark contrast to Mr Ahmadinejad's comments at the UN, the US president said.
"It was offensive. It was hateful. And particularly for him to make the statement here in Manhattan, just a little north of Ground Zero, where families lost their loved ones. People of all faiths, all ethnicities who see this as the seminal tragedy of this generation. For him to make a statement like that was inexcusable," Mr Obama said.
Nearly 3,000 people died on 11 September 2001 when hijacked planes crashed into the twin towers of the World Trade Centre in New York, the Pentagon near Washington and a field in Pennsylvania.
In the interview, Mr Obama also said Afghanistan would not be abandoned, even if the US began cutting its forces there next summer after an extra 30,000 soldiers were deployed earlier this year.
"Now we're seeing Afghan Security Forces trained. We're seeing Afghan police trained. We've got a very effective civilian effort there in order to help build infrastructure and improve the day-to-day lives within Afghanistan," Mr Obama said.
"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".