Image caption The bomb was detonated on a bus full of passengers, local officials say

A bomb on a bus in north-western Pakistan has killed at least 22 people, officials say.

More than 80 were injured in the blast, which occurred in Landi Kotal, a town near the Afghan border.

The bomb appeared to be aimed at members of a pro-government tribe who have formed an anti-Taliban militia.

Landi Kotal is in the Khyber tribal agency, part of Pakistan's tribal north-west which has seen frequent attacks by Taliban militants.

The town lies close to the Torkham crossing into Afghanistan.

"The blast was so powerful, it was heard far and wide and caused damage to nearby buildings," resident Sher Mohammad Shinwari told Reuters news agency.

The bomb was fixed under the vehicle in question and exploded after all passengers had boarded, a local official told the BBC.

Amongst those wounded were children from a nearby school.

The death toll may rise as many sustained serious injuries, officials say.

The attack is believed to have been aimed at members of the Zakha Khel tribe, which has been targeted by militants before.

No group has so far claimed the attack. However, the Pakistani Taliban has claimed responsibility for attacks here in the past - describing them as retaliation for the tribesmen's support for the government.