Abu Yahya al Libi, a Libyan-born top al-Qaeda leader, was killed in a US drone strike in Pakistan earlier this week, a US official said.
US officials said that Abu Yahya had recently been considered by US counter-terrorism experts as the Number two in the core al-Qaeda group led by Ayman al Zawahiri.
Mr Zawahiri has headed the group since al-Qaeda's founder, Osama bin Laden, was killed last year in a US commando raid on his hideout in Pakistan.
US officials said that Libi, who had appeared in al-Qaeda propaganda videos and once escaped from an a US-operated prison in Afghanistan, was a key figure in what remained of the core al-Qaeda network founded by Osama bin Laden, who was killed last year in a US commando raid on his hideout near a Pakistani military academy.
The officials spoke on condition of anonymity to discuss intelligence matters.
In the wake of bin Laden's death, officials said, Mr Zawahri, an Egyptian doctor who had been bin Laden's long-time deputy, became the leader of al-Qaeda's core group, advised and assisted by a small coterie of veteran militants.
US officials said Libi had recently emerged as Zawahri's principal deputy.
"Abu Yahya was among al-Qaeda's most experienced and versatile leaders - operational trainer and Central Shura head - and played a critical role in the group's planning against the West, providing oversight of the external operations efforts," one official said.
Zawahri "will be hard-pressed to find any one person who can readily step into Abu Yahya's shoes - in addition to his gravitas as a longstanding member of AQ's leadership, Abu Yahya's religious credentials gave him the authority to issue fatwas, operational approvals, and guidance to the core group in Pakistan and regional affiliates," the official added.
"There is no one who even comes close in terms of replacing the expertise AQ has just lost."
US officials waited more than 24 hours before spreading word that they were confident Libi had been killed.
In addition to his escape, along with three other militants, from US custody in 2005, he at least once had been reported, prematurely, to have been killed in a US drone strike.
The officials declined to say why they were so confident that Libi was now dead.