Ahmadinejad's comments, made during an address to Intelligence Ministry staff, come amid over its disputed nuclear program. They show that Iran has no intention of toning itself down even with tighter sanctions looming because of its refusal to halt uranium enrichment.
"September 11 was a big lie and a pretext for the war on terror and a prelude to invading Afghanistan," Ahmadinejad was quoted as saying by state TV. He called the attacks a "complicated intelligence scenario and act."
The Iranian president has questioned the official U.S. version of the Sept. 11 attacks before, but this is the first time he ventured to label it a "big lie."
In 2007, New York officials while he was in the city for a U.N. meeting. The president also sparked an uproar when he said during a lecture in New York that the causes and conditions that led to the attacks, as well as who orchestrated them, .
At the time, he also told Iranian state TV the attacks were and that Washington was using Sept. 11 as an excuse to attack others.
He has also questioned the Sept. 11 death toll of around 3,000, claiming the Americans never published the victims' names.
On the 2007 anniversary of the attacks, the names of 2,750 victims killed in New York were read aloud at a memorial ceremony.