Al Qaeda's No. 2 leader issued a new audiotape Tuesday accusing Shiite Iran of spreading a conspiracy theory about who carried out the Sept. 11 attacks to discredit the power of the Sunni terrorist network.

Ayman al-Zawahri, Usama bin Laden's deputy, has stepped up his denunciations of Iran in recent messages in part to depict Al Qaeda as the Arabs' top defense against the Persian nation's rising power in the Middle East.

The increasing enmity toward Iran is a notable change of rhetoric from al-Zawahri, who in the past rarely mentioned the country — apparently in a hopes he would be able to forge some sort of understanding with Tehran based on their common rivalry with the United States. Iran has long sought to distance itself from Al Qaeda.

"Al-Zawahri wanted to work with Iran, but he's deeply disappointed that Iran has not cooperated with Al Qaeda," said Rohan Gunaratna, a terrorism expert and author of "Inside Al Qaeda: The Global Network of Terror."

So now, al-Zawahri "wants to appeal to the anti-Shiite, anti-Iran sentiments in the Arab and Muslim world," said Gunaratna, head of the International Center for Political Violence and Terrorism Research in Singapore.

Al-Zawahri appeared intent on exploiting widespread worry in the Arab world over Iran's influence, particularly in Iraq, to garner support for Al Qaeda. At the same time, he sought to denigrate Iran's ally Hezbollah, which has gained some popularity even among Sunnis in the region for its fight against Israel.

Al-Zawahri's comments came in a two-hour audio posted on an Islamic militant Web site, the second message in weeks in which he answered hundreds of questions sent to the site by Al Qaeda sympathizers and others.

The question-and-answer campaign is a sign of the terrorist network's sophistication in its use of the Web. They show Al Qaeda can post frequent messages from its leaders while keeping in touch with its popular base — all while the leaders remain in hiding, presumably on the Afghan-Pakistan frontier.

The Web is a key tool of Al Qaeda's central leadership to inspire and direct sympathizers at a time when some terrorism experts question how much control they have over Islamic militants. Al Qaeda branches in Iraq and Afghanistan are believed to have close ties with the network's core, but little is known about its level of control elsewhere.

The authenticity of the audio could not be independently confirmed. But the voice sounded like past audios from al-Zawahri, and the posting where it was found bore the logo of Al-Sahab, Al Qaeda's media arm.

Al-Zawahri spoke on a wide range of topics in Tuesday's message. He told a female questioner there were no women in the ranks of Al Qaeda, but praised wives of mujahedeen for their "heroic role in taking care of their homes and children amid the trials of exile."

Female suicide bombers have carried out numerous attacks in Iraq, some of them believed to be by Al Qaeda's branch in the country. Al-Zawahri appeared to mean that no women were among the terrorist network's core leadership.

Al-Zawahri even addressed global warming, saying it showed "how criminal, brutal and greedy the Western Crusader world is, with America at the top."

He predicted that global warming "would make the world more sympathetic to and understanding of the Muslims' jihad (holy war) against the aggressor America."

But in many of his answers, al-Zawahri went out of his way to criticize Iran. He said the Iraqi insurgent umbrella group led by Al Qaeda, called the Islamic State of Iraq, is "the primary force opposing the Crusaders (the United States) and challenging Iranian ambitions" in Iraq.

One questioner asked about the theory that has circulated in the Middle East and elsewhere that Israel was behind the 2001 attacks on the World Trade Center and the Pentagon.

Al-Zawahri accused Hezbollah's Al-Manar television of starting the rumor. "The purpose of this lie is clear — (to suggest) that there are no heroes among the Sunnis who can hurt America as no else did in history. Iranian media snapped up this lie and repeated it," 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 cooperated with the United States in the 2001 U.S. assault on Afghanistan that toppled Al Qaeda's allies, the Taliban.

Al Qaeda has previously claimed responsibility for the Sept. 11 attacks.

In an audiotape last week, al-Zawahri denounced what he called Iran's expansionist plans, saying Tehran aims to annex southern Iraq and Shiite areas of the eastern Arabian Peninsula as well as strengthen ties to its followers in southern Lebanon. He warned that if Iran achieves its goals, it will "explode the situation in an already exploding region."

The rhetoric is a stark change for al-Zawahri, who in the past did not seek to exploit Shiite-Sunni tensions. When the former head of Al Qaeda in Iraq, Abu Musab al-Zarqawi, was waging a campaign of suicide bombings against Shiites in Iraq, al-Zawahri sent messages telling him to stop, fearing it would hurt Al Qaeda's image.

Gunaratna said the change in tone could be because of Al Qaeda's failure to win the release of Al Qaeda figures detained by Iran since the U.S. invasion of Afghanistan, including Al Qaeda security chief Saif al-Adel and two of bin Laden's sons.

Gunaratna said that up to 200 Al Qaeda figures and their families are under house arrest in Iran and that Tehran has rejected Al Qaeda attempts to negotiate their release.

Al Qaeda doesn't have the strength to launch attacks in Iran, but it intends to do so "in the future," he said. "If Al Qaeda becomes strong in Iraq ... Iran believes Al Qaeda in Iraq could become a major threat."