Pakistani police say a bomb has exploded outside a Shi'ite mosque in the country's northwest, killing at least four people.

The bomb went off in the city of Dera Ismail Khan Monday as worshipers were leaving the mosque after evening prayers.

The explosion destroyed parts of the mosque and wounded at least two people.

Police say they suspect the bomb was on a timer. They do not know who planted the device.

Dera Ismail Khan in North West Frontier Province has a history of sectarian violence.

Last month, gunmen killed at least six Shi'ite Muslims in two suspected sectarian attacks in Dera Ismail Khan.

Most Pakistanis are Sunni Muslims, but Shi'ites make up about 20 percent of the country's 160 million people. The communities generally coexist peacefully, but militants from both sides have attacked each other since the 1980s.

Some information for this report provided by AFP, AP and Reuters.