The official told Agence France Presse (AFP) that Washington had a "very high degree of confidence" that Al-Libi was dead, and argued that his demise would severely hamper the capacity of core Al Qaeda leaders to liaise with affiliated groups.
"It is significant," the official said, terming the killing of al-Libi the most important blow against Al Qaeda since US special forces troops swooped into Pakistan last year and killed Al Qaeda leader Osama bin Laden.
Al-Libi, killed in a pre-dawn US drone strike Monday in North Waziristan, a Taliban and Al Qaeda stronghold along the Afghan border, was a Libyan citizen with a $1 million price on his head.
A trusted lieutenant of bin Laden, Libi has appeared in countless Al Qaeda videos and is considered the chief architect of its global propaganda machine.
The official, speaking on condition of anonymity, said that al-Libi had served as the group's "general manager" and had overseen day-to-day operations in Pakistani tribal areas.
Al-Libi was also in charge of contacts with Al-Qaeda affiliates elsewhere, including Al Qaeda in the Arabian Peninsula (AQAP), a Yemen-based offshoot which has launched direct attacks against US targets, the official said.
The official described the killing of al-Libi as a "major blow" to Al Qaeda's core which would further pressure the group's leader Ayman al-Zawahiri