At least nine people have been killed after a bomb exploded in a crowded market in northwestern Pakistan.

The blast in Salarzai town in the Bajaur tribal area also wounded over a dozen people, some of them critically, said Jehangir Azam, a local government administrator.

Twenty others were wounded when the device, planted in a pick-up truck, exploded in Pasht bazaar of Salarzai, known as one of the toughest battlegrounds in Pakistan's fight against the Taliban in the northwest.

"There are nine bodies and 20 wounded brought to hospital," Doctor Khalilur Rehman told the AFP news agency.

Two children aged eight and 13 were among the dead and four children aged  five to 11 were among the injured, he said.

No group has claimed responsibility for the attack, but the Pakistani Taliban have a strong presence in the area.

In recent months the army has carried out several offensives against the extremist groups as they continue to stage attacks on government installations.