Officials say a suicide bomber has struck a mosque in northwestern Pakistan, killing at least 48 people.

The attack occurred as more than 300 people packed into the mosque for Friday prayers in Jamrud, the main town in the Khyber tribal region. Authorities say at least 100 people were wounded in the blast that happened during the Muslim holy month of Ramadan.

TV footage showed a heavily damaged building. Inside the mosque, clothes and sandals were scattered across a blood-splattered floor.

U.S. Secretary of State Hillary Clinton condemned the “slaughter of worshippers” that comes at a “time of celebration and reflection for Muslims throughout the world.”

United Nations Secretary-General Ban Ki-moon said he was “appalled” by the deliberate attack at a place of worship and extended his condolences to the families of the victims.

No one has claimed responsibility, but the Pakistani Taliban has carried out similar attacks in retaliation for the killing of al-Qaida leader Osama bin Laden in May.

Nearly two weeks after the May 2 U.S. raid in Abbottabad, twin suicide bombings killed 80 people outside a paramilitary training center in Khyber-Pakhtunkhwa province.

Khyber has long been a base for Taliban and al-Qaida-linked militants and Pakistani security forces have launched operations to try and stabilize the region.

The United States has also targeted militants in Pakistan's northwest tribal region.

On Friday, a suspected U.S. drone strike killed at least four militants in South Waziristan.

Pakistani officials say missiles hit a house in the Sheen Warsak area.

Meanwhile, political and ethnic violence continued in Pakistan's southern port city of Karachi. Police say at least 52 people have been killed in the country's economic hub since Wednesday.