"The missiles hit an office of Mufti Noor Wali, who was once in charge of training militants for suicide attacks," an unnamed official told the Reuters news agency.

He said it was not known if Wali was among the dead.

Another Pakistani official told the AFP news agency: "It was a US drone attack. We have checked - no Pakistani  aircraft was involved."

Mehsud offensive

Al Jazeera's Imran Khan, reporting from  the capital, Islamabad, said: "The US never confirms whether they are behind these suspected drone strikes.

"But, popularly, it is believed that this is the kind of attack the US has made within this very troubled area of Pakistan before, and they are also likely to be behind these attacks," he said.

In depth

Pakistan army 'taking back' Swat Valley

"Drone strikes are causing much controversy in Pakistan - many people say they have killed innocent civilians and act as a recruitment tool for the Pakistan Taliban."

The attacks were the latest in more than 40 such raids by the US against targets in the area that borders Afghanistan since last August.

At least four people were also reported to have been killed in an attack by Pakistani aircraft on two targets in North Waziristan.

A group of fighters, reportedly allied to Mehsud, had killed at least 16 soldiers in an attack on a military convoy in the area on Sunday. 

The Pakistani military is preparing for an offensive against Mehsud, who has been blamed for a string of deadly suicide attacks across the country that have killed more than 100 people in the past month.

The Taliban leader last week narrowly escaped a suspected US missile attack on a funeral for a number of fighters killed in an earlier drone attack. At least 80 people died in the raid.

Meanwhile, an army transport helicopter carrying security personnel crashed after developing a technical fault, killing 26 people on board, military officials said.

An investigation into the cause of the crash, not far from the main city of Peshawar, was under way, Major-General Athar Abbas, a military spokesman, said.

'Last resistance'

In the Swat Valley, in the North Wrest Frontier Province, the Pakistani military has told Al Jazeera that it has driven out most pro-Taliban fighters after nearly three months of fighting.

Al Jazeera's Kamal Hyder, who was given exclusive access to areas of Swat now controlled by the military, said: "The battle for Swat has been a battle for higher ground.

"The Pakistani military says it is confident that it will be able to root out the militancy problem in the valley.

"They have already regained the high ground overlooking the strategic valleys and now say their men are fighting the last pockets of resistance."

He said he was shown former Taliban strongholds and captured weapons ranging from heavy machine guns and rocket-propelled grenades to large, bloodied knives and cleavers believed to have been used to decapitate prisoners.

The Pakistani government signed a peace deal in February with Sufi Muhammad, a pro-Taliban leader in Swat, in a bid to end 18 months of fighting in the northwest of the country.

But the truce came to an end just two months later, when Taliban supporters moved into neighbouring Buner district in April and the Pakistani military re-started its offensive.

About 2.5 million residents of Swat have fled their homes to avoid the conflict, triggering a humanitarian crisis as relatives and government-run camps struggle to cope.