A suicide bomber targeted Shiite Muslims on two buses being escorted by security forces through a northwestern Pakistan border area rife with sectarian and insurgent violence, killing 12 people Friday.

Tensions between Pakistan's majority Sunni Muslims and Shiites had made the road unsafe for minorities traveling to the nearby Kurram tribal region. Police recently had declared it safe, but Shiites are provided security to travel through it.

Friday's attack only targeted the buses carrying Shiites, police official Akram Ullah said. Security forces escorting them weren't harmed.

The victims were passing through a gas station in the town of Hangu when the lone attacker on foot set off the bomb, Ullah said.

Five people were killed at the scene and seven others died at hospitals, he said.

Pakistan's northwest has been plagued 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 attacks 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.

Sectarian tensions are another matter.

Extremist Sunnis and Shiites have targeted each other's leaders in violence that dates from well before the 2001 terrorist attacks in the United States.

Several of Pakistan's Sunni extremist groups also are allied with the Taliban and al-Qaida, who view Shiites as infidels. The Sunni-Shiite schism over the true heir to Islam's Prophet Muhammad dates to the seventh century.