PESHAWAR, Pakistan -- At least 47 people were killed in the main hall of a mosque in northwest Pakistan on Friday when a suicide bomber detonated himself during midday prayers, local officials said.

There was no immediate claim of responsibility for the attack, which came during the Muslim holy month of Ramadan.

Hundreds had gathered for prayers in the Sunni mosque in the town of Ghundai, in the Khyber region of Pakistan's lawless tribal area. Many were on their way out of the building when the bomb exploded, said a local administrator, Khalid Mumtaz Kundi.

The administrator of the Khyber tribal region, Mutahir Zeb, said in a telephone interview that the number of those killed had reached 47. "This figure may further go up," he said. "Some of the wounded are in critical condition."

The bombing was among the deadliest since twin attacks in a crowded supermarket-hotel complex killed 39 people in Peshawar in June.

The Pakistani Army has waged multiple operations in Khyber aimed at pacifying militants in the region, but it has had limited success. Khyber is also a crucial region for the United States and NATO because a large portion of nonlethal supplies heading to American forces in Afghanistan passes through it.

The village where Friday's attack occurred sits on a highway linking Pakistan and Afghanistan, a main conduit for the oil trucks and NATO convoys that are frequently the targets of militant attacks.

As officials were trying to determine a motive for the suicide bombing at the mosque, a senior government official said that it may have been a reprisal by a group affiliated with the Pakistani Taliban to avenge the death of some of its militants at the hands of a local tribe.

A suicide bomber previously struck a different mosque in the same part of the Khyber region in March 2009. That attack, also during Friday Prayer, left more than 50 people dead.

Witnesses said that the pellets from Friday's blast blew out windows, charred the central hall and cracked one wall of the two-story mosque. "Luckily, the roof didn't come crashing down, or we would have had a huge death toll in the mosque that was jampacked for the main Friday congregation," Mr. Kundi said. Television video showed a heavily damaged building and a grisly scene inside, with clothing and prayer mats scattered across a blood-splattered floor.

"We were offering prayers just when the explosion occurred," Muhammad Sabir, 30, said from a local hospital, where he was being treated for severe burns.

In an attack early Saturday, the authorities said gunmen ambushed a police bus in Karachi, killing four officers. The attack capped a surge in violence in the city in recent days that has killed 60 people. The officers were traveling to a violence-wracked part of the city when the bus was attacked, officials said. Up to 40 other officers were wounded.

Ismail Khan reported from Peshawar, and Kevin Drew from Hong Kong.

First Published: August 20, 2011, 4:00 a.m.