Al Qaeda confirmed on Tuesday that one of the group's most senior figures, veteran militant Abu Yahya al-Libi, had died in a U.S. drone strike earlier this year.
The U.S. government said in June it had killed Libi in Pakistan, dealing the biggest in a series of blows to the group since the raid that killed Osama bin Laden last year.
"I proudly announce to the Muslim umma and to the Mujahideen (holy fighters)... the news of the martyrdom of the lion of Libya Sheikh Hassan Mohammed Qaed," Al Qaeda leader Ayman al-Zawahiri said in a video released on Islamist websites, referring to Libi by his birth name.
Zawahiri's statement was the first acknowledgement by Al Qaeda that Libi had died.
Recently released letters written by bin Laden and captured during the U.S. raid in which he was killed show Libi was one of a handful of al Qaeda operatives who bin Laden relied on to promote the group's case to a worldwide audience of militants, in particular to the young.
A cleric, Libi escaped a high security U.S. prison in Afghanistan in 2005. On at least one previous occasion was he was wrongly reported to have been killed in a U.S. drone strike.
The White House said in June it would be hard for the group to find someone of similar stature to replace him.
Zawahiri's posting said the recording was made during the Muslim fasting month of Ramadan, which ran from mid-July to mid-August, but that it was released to coincide with the anniversary of the Sept. 11 attacks of 2001.
"This liar is trying to fool Americans into believing that he will defeat al Qaeda by killing this person or than person," he said, referring to U.S. President Barack Obama. "But he escapes from the fact that he was defeated in Iraq and Afghanistan."
The United States withdrew most forces from Iraq last year after it led an invasion in 2003 to depose ruler Saddam Hussein, and it is planning a withdrawal from Afghanistan, invaded in 2001 to bring down the Islamist Taliban government.
Zawahiri urged Muslims and particularly Libyans to take revenge for the killing of Libi.
"His blood urges you and incites you to fight and kill the crusaders," he said.
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