A bomb killed eight mourners at the funeral of a slain Shiite cleric Friday in northwestern Pakistan, triggering rioting and heightening sectarian tension in the volatile region.

Police said the attack in the city of Dera Ismail Khan wounded another 28 people and was followed by riots in which a mob burned shops and vehicles and pelted police with rocks. The U.S. this week shifted consular staff from the region because of rising insecurity.

Nuclear-armed Pakistan already faces a wave of Islamist violence that risks destabilizing the country as the West seeks its help in fighting Al Qaeda and the Taliban in neighboring Afghanistan.

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Police said the motive of Friday's bombing was probably sectarian because it followed a string of tit-for-tat attacks between Sunni and Shiite Muslims in the city.

The growing presence of Al Qaeda and the Taliban, both of whom are overwhelmingly Sunni, in northwestern Pakistan has heightened tensions with minority Shiites in the region.

Militants have concentrated their fire on Pakistani security forces and U.S. troops on the other side of the Afghan border. However, Pakistani officials say their ranks have been swelled by banned Pakistani extremist groups that view Shiites as heretics.

Friday's victims were attending the funeral of cleric Allama Nazir Shah Naqvi, who was fatally shot earlier in the day. Police officer Nasir Satti said investigators believed the bomb was detonated by remote control.

Some of the 28 people wounded were in critical condition, said Farid Mehsud, a doctor at a city hospital.

Dera Ismail Khan is about 60 miles from Pakistan's lawless tribal areas, where militants have expanded their influence steadily over the past several years.

American forces in Afghanistan are believed to have staged some 20 missile strikes on militant targets in Pakistan since August, including one on Wednesday that for the first time hit a target beyond the tribal areas.

Pakistan's pro-Western government called in the U.S. ambassador on Thursday to protest the strikes, which it says deepen anti-American sentiment and undermine public support for its own efforts against extremism.

The missile strikes have coincided with several attacks and kidnappings targeting foreigners in the northwest, including the fatal Nov. 12 shooting of an American aid worker in the regional capital, Peshawar.

The U.S. Embassy said in a statement issued late Thursday that it had moved some staff from its consulate in Peshawar to Islamabad while it weighs how to deal with "heightened threat conditions."

Elsewhere in Pakistan's border region, a suicide bomber attacked a mosque where government-backed anti-militant tribesman were praying late Thursday, killing eight, including the head of the group, officials said.

The attack occurred in the Bajur region, where an official said Pakistani troops and warplanes killed 18 militants on Friday.