Ayman al-Zawahiri, al-Qaeda's second-in-command, has vowed revenge for the killing of Abu Laith al-Libi, a top commander of the group in a suspected US attack in Pakistan.
"No chief of ours had died of a natural death, nor has our blood been spilled without a response," al-Zawahiri said in a new video posted on the internet
Al-Libi was reportedly one of Osama bin Laden's top lieutenants in Afghanistan.
He was killed in a suspected US missile strike that killed up to 13 fighters in the north Waziristan border area of Pakistan in January.
"If one of our chiefs passes, another arises in his place," al-Zawahiri said, without making a specific threat.
Al-Libi's involvement with al-Qaeda was highlighted last year by an appearance in a video with al-Zawahiri.
He was the first spokesman to announce that bin Laden had survived the US-led invasion of Afghanistan in late 2001.
According to the US media , al-Libi was believed to be behind a suicide bombing in February 2007 that killed at least 23 people outside the main Bagram base in Afghanistan during a visit by Dick Cheney, the US vice-president.
In October, the US military in Afghanistan named al-Libi among several so-called "mid-level" al-Qaeda and Taliban fighters, and offered a $200,000 reward for his capture.