As in previous years, the tape appears timed to coincide with the anniversary of the 11 September 2001, terror attacks in the United States.
Last year, a video aired just before the second anniversary showed images of al-Zawahri and Osama bin Laden walking together in the mountains. Press reports speculated bin Laden's absence this time indicates he's keeping a low profile amid increased efforts by the United States and others to capture him.
Intelligence agencies are now studying the tape -- though early indications are it is authentic. It's not known when the tape was made, but al-Zawahri does make reference to recent events in Sudan.
On the tape, al-Zawahri -- an Egyptian medical doctor who joined forces with bin Laden in the 1990s -- is wearing a white turban. A machine gun is visible at his side.
In addition to predicting defeat in Afghanistan and Iraq, he signaled Al-Qaeda is planning more attacks in the United States. He called on U.S. President George W. Bush to reinforce security measures, saying, "The Islamic nation which sent you the New York and Washington brigades has taken the firm decision to send you successive brigades to sow death and aspire to paradise."
Al-Qaeda is widely considered responsible for the 2001 attacks in New York and Washington that killed some 3,000 people. Since then, the group has issued several video and audiotapes. Sometimes these appear timed to signal coming attacks, while at other times there's no obvious link.
Al-Jazeera did not say how it obtained the tape. A spokesman for the station said it had broadcast only excerpts from what it said was a 12-minute recording.
The videotape was aired the same day that authorities in two countries announced progress in opposing Al-Qaeda militants.
In Pakistan, army spokesman Major General Shaukat Sultan said a military offensive this week in the border region near Afghanistan had wiped out a militant training camp. The attack, in South Waziristan, killed some 50 militants. About 70,000 Pakistani troops have been deployed in the area as part of the government's efforts to flush out Islamic militants.
In Iraq, interim Prime Minister Iyad Allawi yesterday said Iraqi forces had captured four Al-Qaeda members who had entered the country from abroad. He gave few details in a brief announcement on Al-Arabiyah television. "Four important suspects from Al-Qaeda, who came from abroad [were captured]," he said. "And, hopefully, we will catch more soon. [God] bless you all."
Iraq's U.S.-backed government is battling an insurgency which it says is fuelled partly by foreigners linked to Al-Qaeda.
(compiled from agency and staff reports)
For the latest news on Iraq, see RFE/RL's webpage on "The New Iraq".