A bomb exploded in a Pakistani mosque during the main Friday prayers, killing at least 30 people and wounding more than 70 others in the tribal district of Khyber, an official said.
It was the deadliest attack for two months in the nuclear-armed Muslim country awash with violence blamed on Taliban and Al-Qaeda-linked networks where US special forces killed Osama bin Laden on May 2.
The bomb exploded as hundreds of people had packed into the mosque in the town of Jamrud, 25 kilometres (16 miles) from Peshawar, the main city in the northwest where most of the violence in Pakistan is concentrated.
"At least 24 people have been killed and more than 70 were injured. The bomb blast took place inside the mosque," Mutahar Zeb, the top administrative official of the semi-autonomous district, told AFP.
"There are body parts, there is mutilated flesh so we can't say whether it was a planted bomb or a suicide attack," he added.
Sayed Ahmed Jan, a senior administrative official, initially said seven people died, but with more than 50 people wounded he warned there were fears that the death toll could rise.
Like the rest of the Muslim world, Pakistan is observing the holy month of Ramadan in which faithful fast from dawn to dusk.
Iftikhar Khan, an official at the Hayatabad Medical Complex in Peshawar told AFP that 40 wounded people had been rushed there alone.
There was no immediate claim of responsibility for the attack but bombings blamed on Taliban and Al-Qaeda-linked networks have killed more than 4,550 people since 2007, destabilising the nuclear-armed state.
Friday's bomb was the deadliest since twin attacks killed 39 people in Peshawar, ripping through a crowded supermarket-hotel complex on June 12.