The 33-minute audiotape appeared Tuesday on a Web site known as a clearinghouse for militant Islamic messages. The speaker introduced himself as Musab al-Zarqawi, a Jordanian also known as Ahmed al-Khalayleh who is thought to be a close associate of Osama bin Laden. It was the first tape of any kind attributed to him to be made public.
The tape's authenticity could not be verified. A U.S. official, speaking on the condition of anonymity in Washington, said experts are looking into it, but it was too early to judge its authenticity.
Terrorism experts say that even when such statements cannot be traced to al Qaeda, they serve the group's cause by inspiring sympathizers.
Al-Zarqawi's whereabouts are unknown, but the Web site on which the tape appeared had a transcript heading that said al-Zarqawi was in Iraq.
The tape appeared hours before a Jordanian court convicted al-Zarqawi in absentia and sentenced him to death for the 2002 killing of a U.S. aid official in a terror conspiracy linked to al Qaeda. U.S. officials have offered a $10 million reward for his capture, saying he is trying to build an network of foreign militants in Iraq.
A statement circulating in Iraq and signed by anti-U.S. groups last month claimed al-Zarqawi was killed earlier by American bombs in northern Iraq. A senior U.S. official denied the report of al-Zarqawi's death.
The speaker on the tape claimed responsibility for a March 17 car bombing of a Baghdad hotel that killed seven people. The reference to the car bombing was an indication the tape was made recently.
The speaker also said that his group carried out the assassination of Ayatollah Mohammad Baqr al-Hakim, the leader of Iraq's largest Shiite party, the Supreme Council of the Islamic Revolution in Iraq. Al-Hakim was killed by a car bomb in Iraq on Aug. 29.
Al-Hakim's brother, Abdel Aziz al-Hakim, has said al Qaeda was behind that assassination to try to ignite sectarian conflict.
The speaker also threatened to kill Gen. John Abizaid, head of the Central Command; L. Paul Bremer, the top U.S. administrator in Iraq; and "their generals, soldiers and associates."
One theme of the tape echoed that of a letter U.S. authorities released earlier this year in which al-Zarqawi purportedly wrote to other al Qaeda leaders that the best way to undermine U.S. policy in Iraq was to turn the country's religious communities against each other.
Iraq's Shiite majority was suppressed under toppled Iraqi leader Saddam Hussein, who favored his own Sunni community. Saddam loyalists in heavily Sunni parts of the country and foreign fighters have been blamed for the bulk of attacks against U.S.-led forces in Iraq.
On the tape, the speaker said Shiite Iraqis were not true Muslims and were "the ears and the eyes of the Americans" in Iraq. He called upon Sunni Muslims in Iraq to "burn the earth under the occupiers' feet."