"Al-Zawahri was not there at the time of the attack, " the official said, after US intelligence sources in Washington had earlier said the airstrike that killed at least 18 people in northern Pakistan had targeted Osama bin Laden's deputy.

Quoting intelligence sources, US media said it was a CIA raid. The US military says it is not aware of any operations taking place in the area. The raid took place in the village of Damadola in the Bajaur tribal area, about 200km northwest of Pakistan's capital, Islamabad, and about 7km inside Pakistan's border.

Jets reportedly fired missiles at a particular housing compound in the village, which may have been an al-Qaeda hideout. Some unconfirmed accounts said foreigners were killed in the attack, and their bodies removed.

Zawahiri, seen as Osama Bin Laden's second-in-command, has eluded capture since the US toppled the Taliban in Afghanistan in 2001 despite a $25m bounty on his head.

He has recently come to be regarded by the US as the operational head of al-Qaeda because Bin Laden is not in a position to run the network.

US television networks, quoting Pakistani military sources, say five of those killed in the strike were thought to be senior al-Qaeda members.

Pakistani information minister Sheikh Rashid told reporters the authorities were still investigating the reports and he could confirm nothing.

Maj Gen Shaukat Sultan, spokesman for President Pervez Musharraf, was also unable to say whether alZawahiri had been targeted.

"We are investigating and as of now we are not in a position to say yes or no, " he said.

Associated Press news agency later quoted two separate officials, unnamed, as saying that al-Zawahiri had not been killed in the attack.

The US defence department has denied its forces carried out any attacks in the area. "There is no reason to believe the US military is conducting operations there, " Lt Col Todd Vician said.

US television network NBC quoted Pakistani sources as saying the strikes were probably carried out by unmanned CIA Predator drones, which fired up to 10 missiles.

The US has about 20,000 troops in Afghanistan, but Pakistan does not allow them to operate across the border.

Pakistan has about 70,000 troops in the border region.