Al-Qaida's number two leader, Ayman al-Zawahri, has vowed to avenge the killing of a top al-Qaida commander in Pakistan last month.

Zawahri made the threat in a new video released Wednesday on the Internet.  In the video, he pays tribute to Abu Laith al-Libi, who was believed to be the terror network's third-most important figure after Osama bin Laden and Zawahri himself.

The Libyan-born Libi was killed in what Pakistani intelligence officials say was a U.S. missile strike in Pakistan's North Wazaristan tribal region in late January.

Zawahri says in the video that no al-Qaida chief has died a natural death and no al-Qaida blood has been spilled without a response.  He did not elaborate.

U.S. officials say Libi planned a suicide bombing at the main U.S. military base in Afghanistan in February 2007 during a visit by U.S. Vice President Dick Cheney. The attack killed 23 people.

The Zawahri video was posted on an Islamist Web site that usually releases al-Qaida messages.  The U.S. government has not verified its authenticity.

The video is the first public message issued by Zawahri this year.  He issued 15 messages in 2007.

Al-Qaida's media branch, al-Sahab, produced the 10-minute video, which carried English subtitles.

Some information for this report was provided by AFP, AP and Reuters.