Media playback is unsupported on your device Media caption The BBC's Orla Guerin says Hangu has a "history of sectarian violence'"

At least 21 people have been killed and dozens injured in a suicide bomb attack outside a Shia mosque in the north-western Pakistani town of Hangu, reports say.

The attack took place as worshippers were leaving Friday prayers.

The town of Hangu is close to Pakistan's semi-autonomous tribal regions, which have a strong Taliban and al-Qaeda presence.

The town is also known for sectarian violence against Shias.

No group has yet said it carried out the attack.

Shops damaged

Local police chief Mian Mohammad Saeed said the bomb went off at one of the exits of the Shia mosque leading to the Pat Bazaar.

Sunni and Shia Muslims Muslims are split into two main branches, the Sunnis and Shias

The split originates in a dispute soon after the death of the Prophet Muhammad over who should lead the Muslim community

There are also differences in doctrine, ritual, law, theology and religious organisation

The great majority of Muslims are Sunnis

Pakistan - where Shias are a minority - has a history of sectarian bloodshed dating back to the 1980s Pakistan's evolving sectarian schism Formidable power of Pakistan's anti-Shia militants

"It was a suicide attack which targeted Shias but Sunni Muslims also fell victim since their mosque and some shops were also very close to the site," he said.

TV footage showed several small shops damaged and a wall pitted with shrapnel.

A motorcycle was packed with 7kg of explosives in a narrow street, police said.

Security forces have cordoned off the area, and the injured have been taken to a local hospital.

"As soon as I reached the mosque exit, a huge blast rocked the area. Many people fell on me with the impact of the blast," eyewitness Muzammil Hussain told AFP news agency.

"I saw red and bloodied pieces of human flesh everywhere. It was a scene I'd never seen in my life before."

Hangu is a Sunni majority town, but is located on the main route to the mainly Shia Kurram region.

Pakistan has experienced worsening sectarian violence in recent years.

In November, 23 Shia Muslims were killed by a bomb in the city of Rawalpindi, and last month 19 pilgrims died in an attack on a bus convoy in Baluchistan.

Then on 10 January, more than 80 people died in twin blasts in a snooker hall in a Shia area of the city of Quetta.