A suicide bomber killed 48 worshipers Friday at a mosque in a northwestern Pakistani tribal region long scarred by militant attacks and sectarian clashes, officials reported.

The bombing in the Khyber region near the Afghan border highlighted the persistence of violence in this majority-Muslim nation despite the observance of Ramadan, a month-long period of spiritual introspection. The Sunni mosque in the village of Ghundi was packed with worshipers for Friday prayers, eyewitnesses said.

No group immediately claimed responsibility for the attack.

Eyewitness and media reports said a boy dressed in black who appeared to be 15 or 16 years old entered the mosque through a window as services were ending and exploded a suicide vest packed with ball bearings. Local and hospital officials said that at least 100 people were injured and that the death toll could rise.

“Whoever did it in the holy month of Ramadan cannot be a Muslim,” Saleem Khan, a survivor, told the Associated Press from a hospital bed in Peshawar, the major northwestern city, to which many of the injured were taken. “It is the cruelest thing any Muslim would do.”

An estimated 300 worshipers were inside the mosque when the explosion destroyed the building’s doors and walls, but early reports of a roof collapse appeared to be unfounded

Secretary of State Hillary Rodham Clinton condemned the bombing.

“The slaughter of worshippers as they gathered at a mosque for Ramadan’s Friday prayers underscores the brutality of those who would target civilians during a time of celebration and reflection for Muslims throughout the world,” she said in a statement.

The Khyber tribal area is strategically important because military supplies pass through it en route to U.S. forces in Afghanistan. Militants have mounted numerous attacks on oil tanker trucks and other supply vehicles in recent years.

The region also has been the scene of deadly fighting between two Sunni sects, the conservative Deobandi and more moderate Berelvi.

Investigators said they were exploring various motives for the attack, including a feud between the Pakistani Taliban and a local tribe. Several outraged and bereaved villagers said the people at the mosque were not involved in any such disputes.

“There are various militant and sectarian outfits in the region, but we are neutral and abstain from the ongoing rifts,” a village elder, Malik Khani Jan, said by telephone. “This is a condemnable act on the poor and innocent worshipers in the holy month of fasting.”

In the past year, scores of people have died in religiously motivated attacks at shrines and mosques throughout the country.

Friday’s suicide bombing comes at a time when violence is also raging in Karachi, Pakistan’s financial center and its largest city. In recent days, an estimated 40 people have been killed in fighting among ethnic and political groups and criminal gangs — clashes marked by torture, kidnappings and execution-style shootings.

Special correspondent Haq Nawaz Khan in Peshawar contributed to this report.