The comments reflected the increasing criticism by Ayman al-Zawahri, al-Qaeda's No 2, against Iran, which he has accused in recent messages of seeking to extend its power in the Middle East, particularly in Iraq and through its Hezbollah allies in Lebanon.

Zawahri spoke in a two-hour audio clip posted on an Islamic militant website, in which he replied to hundreds of questions submitted on the internet by al-Qaeda sympathisers. One of the questioners asked about the theory that has circulated that Israel was behind the 2001 attacks on the World Trade Centre in New York and the Pentagon.

Zawahri accused Iran-backed Hezbollah's al-Manar television of starting the rumour. "The purpose of this lie is clear – (to suggest] that there are no heroes among the Sunnis who can hurt America as no-one else did in history," he said.

"Iran's aim here is also clear – to cover up its involvement with America in invading the homes of Muslims in Afghanistan and Iraq," he added. Iran co-operated with the United States in the 2001 assault on Afghanistan that toppled al-Qaeda's allies, the Taleban.

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