Masri reportedly masterminded attacks on US-led troops in Afghanistan
Abu Obaidah al-Masri, the militant chief believed to be a senior planner of al-Qaeda's attacks in Afghanistan, has died, US intelligence sources say.
They said they believed Masri, a major operational figure within al-Qaeda linked to attacks in the UK and elsewhere, died of "natural causes".
"The sense is that he is dead," a US official said.
A US official told the BBC that Masri had apparently died within the last two months, probably of hepatitis.
The Egyptian-born militant was described by both US and British counter-terrorism officials as head of external operations for al-Qaeda's core leadership.
A former military commander in Afghanistan, Masri is believed to have directed attacks on US-led forces after the Taleban were ousted in 2001, before turning his attention to high-impact attacks on Western populations, says the BBC's security correspondent, Frank Gardner.
Thought to have been hiding out in Pakistan's tribal areas before he died, counter-terrorism officials believe Masri was so important to al-Qaeda that his successor has yet to be appointed, our correspondent adds.
He was such a mysterious figure that even his real name was a carefully veiled secret - Abu Obaidah al-Masri is an Arabic nom de guerre meaning "Father of Obaidah the Egyptian".
Masri had escaped at least two assassination attempts in the past.
Pakistani officials reported he had been killed in a 2006 missile strike on a Pakistani village near the Afghan border, only to say later that he had not been in the village at the time.
Another missile attack on a different Pakistani village later in 2006 also missed its target.
MJ Gohel, a terrorism specialist at the Asia Pacific Foundation, an independent intelligence think tank based in London, said Masri was a known explosives expert.
He was also a key figure in spreading Islamic militancy to Europe by bringing young Muslims with Western backgrounds to Pakistan for training, said Mr Gohel.
"The terror trail keeps leading back to Pakistan, and Masri was an essential part of al Qaeda's headquarters in that country," he told Reuters news agency.