A bomb blast killed around 40 worshippers attending Friday prayers at a mosque in an area of northwest Pakistan, a senior official in the area told Reuters.
"The death toll is 40. We have no idea as yet how many have been wounded," said Atif-ur-Rehman, the senior-most government administrator in the Upper Dir district, close to Swat Valley.
Umer Rehman, a resident of Hayagai village, around 200 km (125 miles) northwest of Islamabad, said 30 bodies had been identified.
The latest blast is the ninth bomb attack in the country since the army launched a major offensive against the Taliban in neighbouring Swat valley.
The army has been conducting a major offensive against the Taliban
The military says more than 1,200 fighters and 90 soldiers have been killed since late April.
US special envoy Richard Holbrooke Holbrooke met Prime Minister Yusuf Raza Gilani and Foreign Minister Shah Mehmood Qureshi on Friday, and had met Zardari's main rival, former prime minister Nawaz Sharif, a day earlier. He was also said to have met Kayani, though officials declined to confirm any meeting.
Holbrooke had said he would discuss with Pakistani leaders the "next phase" of holding, securing and rehabilitating Swat after the operation against the valley was completed.
There is speculation that once Swat is under control, the army could switch its focus to South Waziristan.
The army retook Swat's main town of Mingora last week.
Holbrooke has said the United States aimed to give Pakistan $200 million, in addition to $110 million already pledged, to help deal with the humanitarian crisis caused by US-backed offensive.