Al-Qa'ida's second in command in Pakistan was in a house hit by a US drone strike, according to intelligence officials, but they do not know whether he was killed.
US officials said they were targeting Abu Yahya al-Libi in yesterday's strike in the North Waziristan tribal area and were "optimistic" he was killed.
Pakistani intelligence officials said militants and local residents reported al-Libi was in the house.
A local Taliban chief said al-Libi's guard and driver were killed, but the al-Qa'ida commander was not there.
A vehicle used by al-Libi was destroyed in the strike in Khassu Khel village, according to one of the Pakistani officials.
Agents intercepted a militant phone call indicating an Arab was killed in the attack, but it is unclear if they were talking about al-Libi, who was born in Libya, said the official.
A local Taliban chief said al-Libi's guard and driver were killed, but the al-Qa'ida commander was not there. Al-Libi did survive a previous strike, said the Taliban chief.
If al-Libi is confirmed killed, he would be the latest of more than a dozen senior commanders removed in the clandestine US war against al-Qa'ida since Navy SEALs killed Osama bin Laden last year.
The White House maintains a list of terrorist targets to be killed or captured, compiled by the military and the CIA and ultimately approved by the president.
The US has stepped up drone strikes in Pakistan recently, carrying out seven in less than two weeks. The flurry follows a relative lull driven by tensions between Washington and Islamabad over American air strikes last year that killed 24 Pakistani soldiers.
Pakistan seized the opportunity to renegotiate its relationship with the US and demanded Washington stop drone strikes in the country - a demand the US has ignored. The attacks are unpopular in Pakistan because many people believe they mostly kill civilians, an allegation disputed by the US.
Members of the Pakistani government and military have supported the strikes in the past, but that co-operation has come under strain as the relationship between the two countries has deteriorated.
The US State Department's Rewards for Justice programme had set a million dollar reward for information leading to al-Libi, who had filmed numerous propaganda videos urging attacks on US targets after he escaped a prison at Bagram Air Base in Afghanistan in 2005.
Al-Libi took the second-in-command spot when Egyptian-born Ayman al-Zawahri took charge of al-Qa'ida after Bin Laden's death. As al-Qa'ida's de facto general manager, al-Libi is responsible for running the group's day-to-day operations in Pakistan's tribal areas and manages outreach to al-Qa'ida's regional affiliates.
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