WASHINGTON — 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, U.S. officials said Saturday.

The Libyan national had been the network's operational leader before rising 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."

Since bin Laden's death, al-Qaida's structure has been unsettled and U.S. officials have hoped to capitalize on that. The more uncertain the leadership, the harder it is for al-Qaida to operate covertly and plan attacks.

Bin Laden's longtime deputy, Ayman al-Zawahiri, is running the group but is considered a divisive figure who lacks the founder's charisma and ability to galvanize al-Qaida's disparate franchises.

A U.S. official, who spoke on condition of anonymity to summarize the government's intelligence on al-Rahman, said al-Rahman's death will make it harder for Zawahiri to oversee what is considered an increasingly weakened organization.

"Zawahiri needed Atiyah's experience and connections to help manage al-Qaida," the official said.

Al-Rahman was killed Aug. 22 in the Pakistani tribal region of Waziristan, according to a senior administration who also 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.

Rahman has been thought to be dead before. Last year, there were reports that Rahman was killed in a drone strike but neither senior U.S. administration officials nor al-Qaida ever confirmed them.

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