US missiles pounded a Pakistani Taliban commander's hideout in the lawless tribal area of South Waziristan on Friday and casualties were feared, officials said.
Security sources gave varying accounts of five or possibly up to 13 people dead, but those reports could not be immediately confirmed.
"Three missiles hit the hideout of Taliban commander Noor Wali. Casualties are feared, but details are not immediately available," one official told AFP on condition of anonymity as he was not authorised to speak to the media.
Wali is a close ally of Pakistani Taliban warlord Baitullah Mehsud, who has a five-million-dollar price on his head.
Wali's compound was hit in the village of Kokat Khel in South Waziristan, which lies on the border with Afghanistan.
Pakistani troops have been pressing a two-month battle to dislodge Taliban insurgents from northwest districts and the military has said it will open a second front in the tribal regions to track down Mehsud.
"It was a US drone attack. We have checked -- no Pakistani aircraft was involved in this incident," a Pakistani military official said.
"There were missile strikes in two places. The targets were militant hideouts. We are upchecking casualties," another Pakistani security official told AFP on condition of anonymity.
The United States military does not, as a rule, confirm drone attacks, but its armed forces and the Central Intelligence Agency operating in Afghanistan are the only forces that deploy drones in the region.
Washington has branded Pakistan's rugged northwest tribal belt as the most dangerous place in the world for Americans, saying Al-Qaeda and Taliban rebels are plotting attacks on Western targets from militant hideouts there.
Pakistan publicly opposes US strikes, saying they violate its territorial sovereignty and deepen resentment among the populace. Since August 2008, at least 44 such strikes have killed more than 440 people.
Friday's attack came 10 days after drone aircraft hit Taliban positions as hundreds gathered for a funeral in Mehsud's South Waziristan stronghold.
Security officials and Taliban militants wildly differed on casualties from that attack in a remote, mountainous area. Pakistan's army said between 20 and 30 people were reported killed but others said up to 65 people died.
The United States has adopted a new strategy to defeat Islamist extremists, putting Pakistan at the heart of the fight against Al-Qaeda and sending an extra 21,000 US troops to neighbouring Afghanistan to battle the Taliban.
Washington alleges Al-Qaeda and Taliban rebels who fled Afghanistan after the 2001 US-led invasion are holed up in South Waziristan.