A suicide bomber detonated his explosives outside a Shiite mosque in northwestern Pakistan as worshippers were leaving Friday prayers, killing at least 21 people and wounding 36 in the latest apparent sectarian attack in the country, police said.

Shiite Muslims in Pakistan have increasingly been targeted by radical Sunnis who consider them heretics, and 2012 was the bloodiest year for the minority sect in the country's history.

The attack on the mosque took place in the town of Hangu in Khyber Pakhtunkhwa province, which has experienced previous clashes between the Sunni and Shiite communities that live there.

The bomber staged his attack at one of the mosque's exits leading to a bazaar, said Hangu police chief Mian Mohammad Saeed. At least 21 people were killed and 36 wounded. One policeman who was guarding the mosque was killed and another was injured, he said.

Most of the dead and wounded were Shiites, but some of the casualties were also from the country's majority sect since there is a Sunni mosque nearby, said another police officer, Naeem Khan.

Hangu has experienced conflict in the past between the Sunni and Shiite communities that live in the town. Both sides have attacked each other's shops and burned them.

The worst sectarian violence in Pakistan in recent years has been in southwestern Baluchistan province, which has the largest concentration of Shiites in the country. A twin bombing last month at a billiards hall in the provincial capital, Quetta, killed 86 people, most of them Shiites.

According to Human Rights Watch, more than 400 Shiites were killed in targeted attacks in Pakistan in 2012, including over 120 in Baluchistan.

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Associated Press writer Hussain Afzal contributed to this report from Parachinar, Pakistan.