Pakistan is investigating an apparent missile strike on a village near the Afghan border which is reported to have targeted the deputy leader of al-Qaeda.
Al-Zawahiri has eluded US capture since 2001 despite a $25m bounty
At least 18 people were killed in the raid but it remains unclear if Ayman al-Zawahiri was among them.
The Pakistani information minister said he was unable to confirm the attack.
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 Bajaur tribal area.
The BBC's Zaffar Abbas in Karachi says tribesmen there are convinced the strike was the work of the Americans and are very angry at the cross-border attack.
Jets reportedly fired missiles at a particular housing compound in the village, which may have been an al-Qaeda hideout, our correspondent says.
Some accounts say 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 Taleban 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, our correspondent adds.
US television networks, quoting Pakistani military sources, say five of those killed in the strike are 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.
Major General Shaukat Sultan, spokesman for President Pervez Musharraf, was also unable to say whether Zawahiri had been targeted.
He said: "We are investigating and as of now we are not in a position to say yes or no."
Associated Press news agency later quoted two separate officials, unnamed, as saying that 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 told AFP news agency.
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 raid took place in the village of Damadola in the Bajaur tribal area, about 200km (125 miles) north-west of Pakistan's capital, Islamabad.
The village is about 7km (4.5 miles) inside Pakistan.
Locals earlier claimed the missiles were launched from neighbouring Afghanistan in the early hours of Friday, but officials there deny it.
Shah Zaman said two sons and a daughter were killed in the strike
Last weekend, Pakistan protested after eight died in nearby Waziristan in what it said was cross-border firing.
Afghan and US-led coalition forces believe Taleban-led militants take advantage of the porous border.
Reporters who reached Damadola spoke of three houses hundreds of metres apart that had been destroyed.
Shah Zaman said he lost two of his sons and a daughter. "I ran out and saw planes. I ran toward a nearby mountain with my wife. When we were running we heard three more explosions. I saw my home being hit.
"I don't know who carried out this attack and why. We were needlessly attacked. We are law-abiding people."
A Pakistani intelligence official told Reuters news agency that Damadola was the stronghold of a banned pro-Taleban group, the Tehrik-e-Nifaz-e-Shariat-e-Mohammadi.
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.