At least 48 people were killed and 150 injured when a suicide bomber detonated explosives during Friday prayers at a mosque in the Khyber region of northwest Pakistan, according to news reports from the area and residents.

Tariq Hayat , the chief administrator of the region close to the Afghanistan border, told Geo TV that “so far we have counted 48 bodies.”

Residents said that about 250 to 300 worshipers were in the mosque at the time of the attack and that the death toll could rise sharply.

“It was a suicide bombing,” Mr. Hayat said. The bombing came as President Obama prepared to announce his new strategy to bolster American forces in Afghanistan and for the first time set benchmarks for progress in fighting Al Qaeda and the Taliban in both Afghanistan and Pakistan. Quoting witnesses, Mr. Hayat said the bomber had mingled with the congregation and blown himself up as prayers were about to begin.

Mosques across the Islamic world are especially crowded on Fridays and the scale of the attack dwarfed other recent attacks in the region.

One survivor, who uses only the name Arman, said that just as a cleric began to intone the words “God is great,” we heard a powerful explosion. There were blood and limbs all around. I had never seen such a horrendous scene in my whole life.”

The blast brought the roof of the two-story building tumbling onto the first floor ceiling, which collapsed, residents said.

Mr. Hayat blamed the militant group Tehrik-i-Taliban Pakistan for the explosion.

“Their aim is to spread terror and any place where they cause maximum casualties is a legitimate target for them,” Mr. Hayat said in a telephone interview. Tehrik-i-Taliban is a militant group headed by Baitullah Mehsud, whose forces are battling the Pakistani Army in increasingly large swaths of the country. Its training camps have recently been attacked by American drones.

The mosque was about 12 miles from Peshawar on the main highway between Afghanistan and Pakistan which is used as a principal supply route for NATO and American forces in Afghanistan.

The mosque was often used by worshippers passing by on the main highway.

After the attack, dozens of bodies were taken to a hospital in the neighboring town of Jamrud, while relatives removed others for burial, residents said.

Among the dead were 10 police officers and four paramilitary soldiers from a nearby checkpoint but Mr. Hayat denied they could have been the target.

Many of the dead were civilians, including children, with a handful of local security forces among them, residents reported.

Television news footage from the attack showed men in long robes and white skullcaps standing atop a wall overlooking the wreckage of the mosque and carrying corpses from the debris. A minaret with a loudspeaker jutted from the rubble and other people clawed through rubble seeking bodies or survivors.

Initial reports did not indicate any claim of responsibility for the attack, which came as news emerged that Taliban leaders in Pakistan had agreed to close ranks with their Afghan counterparts to confront a planned increase of 17,000 in the number of American troops in Afghanistan.

The United States also plans to send an extra 4,000 soldiers to train Afghan security forces. The increases will bring the overall American deployment in Afghanistan to about 60,000. The Times of London reported on Friday that Britain was considering sending 2,000 soldiers to support the American reinforcement, in addition to the 8,300 troops it already has in Afghanistan.

The Khyber area is known for bitter feuds between rival tribes and groups of militants.

Most of the 158 wounded were brought to the main hospitals in Peshawar. A doctor at one of the hospitals said that they had declared an emergency to deal with the high number of casualties.

Suicide Bomber Kills 48 in Pakistan