Senior al-Qaeda leader Abu Yahya al-Libi was killed in a drone strike in Pakistan on Monday, a US official has told the BBC.
US officials said Libi was the target of a drone strike which hit a volatile tribal area of Pakistan's north-west, killing 15 suspected militants.
There has been no confirmation of his death from sources in Pakistan.
According to officials, he played a critical role in the group's planning against the West.
"There is no one who even comes close in terms of replacing the expertise al-Qaeda has just lost," the US official told the BBC.
The source added that al-Qaeda's leadership "will be hard-pressed to find any one person who can readily step into [Libi's] shoes".
But the drone strike sparked anger in Pakistan and the foreign ministry lodged a formal protest with a US diplomat in Islamabad.