At least 16 people have been killed and 17 wounded by a bomb at a marketplace in Kalaya, in Pakistan's Orakzai tribal region, officials say.

The blast went off close to a number of government buildings.

It is unclear if officials or members of an anti-Taliban tribe were the intended target.

Orakzai is an area where Taliban insurgents are active and battle government forces as well as local anti-Taliban militias.

Pakistan's military has carried out a series of offensives against Taliban militants in the north-west in recent years, but the militants have proved difficult to defeat and have continued to carry out regular attacks against security forces and civilians in the area.

No group has claimed responsibility for the latest bombing.

Government spokesman Fazal Qadir told the BBC that most of the dead were worshippers leaving a mosque in the main square in Kalaya following Friday prayers.

Police say that the blast took place near a video, CD and mobile shop surrounded by small kiosks selling tea and other edibles.

Officials told Reuters that nine militants were killed after jets bombed their hideout following the attack.

The Pakistan army launched a major offensive in Orakzai in the spring of 2010, and in October of that year announced that it had cleared much of the area it described as "the Taliban's centre of gravity in Pakistan".

Correspondents say that clearing out militant sanctuaries near the Pakistani border with Afghanistan is seen as crucial to US efforts to bring stability to Afghanistan ahead of the scheduled Nato pull-out in 2014.

"According to our initial information, the bomb was planted near the video CD and mobile shop surrounded by small kiosks selling tea and other edibles," local official Mehmood Aslam told the AFP news agency.

Last October at least four people have been killed in a bomb blast in a market in Orakzai.