Al-Qaida's second-in-command, Atiyah Abd al-Rahman, has been killed in Pakistan, delivering another big blow to a terrorist group that the U.S. believes to be on the verge of defeat, a senior Obama administration official said Saturday.

The Libyan national who was the network's former operational leader rose to al-Qaida's No. 2 spot after the U.S. killed al-Qaida leader Osama bin Laden during a raid on his Pakistan compound in May.

Defense Secretary Leon Panetta said last month that al-Qaida's defeat was within reach if the U.S. could mount a string of successful attacks on the group's weakened leadership.

"Now is the moment, following what happened with bin Laden, to put maximum pressure on them," Panetta said, "because I do believe that if we continue this effort we can really cripple al-Qaida as a major threat."

Al-Rahman was killed Aug. 22 in the lawless Pakistani tribal region of Waziristan, according to the official said, who insisted on anonymity to discuss intelligence issues.

The official would not say how al-Rahman was killed. But his death came on the same day that a CIA drone strike was reported in Waziristan. Such strikes by unmanned aircraft are Washington's weapon of choice for killing terrorists in the mountainous, hard-to-reach area along the Pakistan-Afghanistan border.

Al-Rahman, believed to be in his mid-30s, was a close confidant of bin Laden and once served as bin Laden's emissary to Iran.

Al-Rahman was allowed to move freely in and out of Iran as part of that arrangement and has been operating out of Waziristan for some time, officials have said.

Born in Libya, al-Rahman joined bin Laden as a teenager in Afghanistan to fight the Soviet Union.

After Navy SEALs killed bin Laden, they found evidence of al-Rahman's role as operational chief, U.S. officials have said.