A suicide bomber on foot targeted a group of Shiite Muslims on two buses in northwestern Pakistan on Friday, killing at least five people and wounding 25 others in the latest violence to rock the Afghan border region, police said.

The attack may have been motivated by sectarian tensions between Pakistan’s majority Sunni Muslims and minority Shiites. However, the area also is near al-Qaeda and Taliban-controlled parts of Pakistan’s tribal belt.

The victims, which included women, were at a gas station in the town of Hangu, police official Omar Hayat said. The wounded were taken to nearby hospitals, as police investigated the scene.

Pakistan’s northwest has been bedeviled for years by Islamist extremist violence fueled by anger over the war in Afghanistan and Islamabad’s alliance with Washington. An army offensive that began in October against the Pakistani Taliban spurred a bloody wave of attacks across the country that killed more than 600 people.

But with the exception of a few attacks on northwest police stations, violence appears to have subsided in recent weeks, an indication that the army operation in the South Waziristan tribal region may be having an impact.

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