Media playback is unsupported on your device Media caption The BBC's Orla Guerin: "They said their main target was an officer in the police force"

A suspected suicide blast in Pakistan's north-western Bajaur district has left at least 20 people dead, officials say.

The blast is reported to have targeted security forces close to a market place in Khar, the tribal area's main town. Most of those killed were bystanders.

The Pakistani Taliban say it was a suicide bombing - officials have yet to confirm the cause of the blast.

Bajaur, close to the Afghan border, has seen heavy fighting between Pakistani troops and Taliban militants.

The army has repeatedly declared the area to be secure, only for the militants to prove them wrong, the BBC's Orla Guerin in Islamabad says.

The bomber was on foot and detonated an explosive vest when he reached a police checkpoint, some reports say.

Reports say several places in the busy market were damaged by the blast.

Five of those killed were tribal policemen and one of them had received a presidential award for bravery in fighting militants, reports said.

The local Taliban group, led by Maulvi Faqir, has said that it carried out the attack to target a commander in the security forces who killed fellow insurgents last year.

A curfew has been imposed in the area, police say. There have been several deadly blasts across Pakistan's restive tribal districts this year.

In March, at least 20 people were killed when a suicide bomb targeted a mosque in the Khyber area.