The U.S. official emphasized Libi's importance to the terror group.
There is no-one who even comes close in terms of replacing the expertise al-Qaida has just lost, he told the BBC, adding that al-Qaida's leadership will be hard-pressed to find any one person who can readily step into [Libi's] shoes.
Libi, an Islamic scholar from Libya, was believed to be al-Qaida's second-in-command, after the leader Ayman al-Zawahiri, who took over following the assassination of Osama bin Laden in May 2011.
The drone strike also killed 15 other suspected militants, according to BBC.
However, Pakistan, which expressed its anger over the drone strike mission, has not officially confirmed the deaths.