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

The Libyan national who was the network's former operational leader, rose up the chain of command after the US killed al-Qaida leader Osama bin Laden during a raid on his Pakistan compound in May.

US defence secretary Leon Panetta said last month that the defeat of al-Qaida was within reach if the US 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."

Rahman was killed on 22 August in the lawless tribal region of Waziristan in Pakistan, according to the official, who insisted on anonymity.

The official would not say how Rahman was killed, but his death came on the same day a drone strike in Waziristan was reported. Such strikes by unmanned aircraft are Washington's counter-terrorism weapon of choice in the mountainous, hard-to-reach area along the border with Afghanistan.

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

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

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

US officials have said that after Bin Laden's death, the US navy found evidence of Rahman's role as operational chief.

Bin Laden's long-time deputy, Ayman al-Zawahiri, is running al-Qaida but is considered a divisive figure who lacks the founder's charisma and ability to galvanise the group's disparate franchises. The anonymous US official said 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," he said.