Al-Qaeda number two Ayman Al-Zawahiri vowed to avenge the recent death of one of his group's top operatives, in a videotape message broadcast on an Islamist website on Wednesday.
Zawahiri made the threat in a eulogy to Abu Laith al-Libi, who was killed in a missile strike on January 28 in the Pakistani tribal area of north Waziristan.
"No chief of ours has died a natural death. Nor has our blood been spilled without a response," he said in the 10-minute recording which had English subtitles and was posted on the Al-Ekhlaas website.
Pakistani security officials said the Libyan militant was Osama bin Laden's operational number three, functioning as the key linkman between al-Qaeda and Taliban insurgents in Afghanistan.
Among other attacks, he was blamed for a bloody suicide bombing at the main U.S. military base in Afghanistan in February 2007 which occurred during a visit by U.S. Vice President Dick Cheney.
Cheney was unscathed -- but the audacious attack earned al-Libi fifth place on a U.S. Central Intelligence Agency list given to Pakistani security sources, together with a five-million-dollar (3.3-million-euro) bounty.
Zawahiri, who appeared in front of a wall of books and a rifle, said al-Libi was "one of the mountains of jihad in this era... he departed after making the Americans drink draughts of death."
Zawahiri last made a public declaration in December, when in an audiotape he lashed-out at a U.S.-hosted Middle East peace conference, calling it a betrayal of the Palestinians.