Pakistan officials say gunmen in the southern port city of Karachi have ambushed a bus carrying police commandos, killing at least four officers and wounding more than 30 others.

Authorities say the commandos were dressed in plain clothes when they were attacked late Friday in the eastern Korangi district of Karachi.

Police said earlier Friday at least 52 people have been killed in political and ethnic violence in Karachi since Wednesday.

Elsewhere, a suicide bomber struck a mosque in northwestern Pakistan during Friday prayers, killing at least 48 people.

The attack occurred as more than 300 people packed into the mosque 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.

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.

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.