CNN quoted an anonymous U.S. official as saying the attack took place Monday in North Waziristan region, near the Pakistan- Afghanistan border, and at least six missiles were fired at a militant compound near the town of Mir Ali. It was the 21st suspected U.S. drone strike in Pakistan this year.

The official was quoted as saying al-Libi's death marks one of the most significant blows to al-Qaeda since Osama bin Laden was killed over a year ago. He rose to second-in-command behind Ayman al-Zawahiri, who replaced bin Laden as the group's leader.

The official was also quoted as saying al-Libi played a critical role in the group's planning against the West, providing oversight of the external operations efforts, and he would be hard to replace.

A Libyan, al-Libi was captured in 2002 and held by U.S. forces in Bagram detention facility in Afghanistan, but escaped on the night of July 10, 2005. U.S. government has a one million dollar bond on his head.