The explosion happened as the bus passed over a bridge on the outskirts of the city. A witness saw a large crater in which the mangled wreckage of the bus was lying, while the scene was spattered with blood.

'Eleven dead bodies and 11 injured have now been brought here,' Mr Khan Abbas, a police official deployed to Peshawar's main Lady Reading hospital, said.

The police chief of North West Frontier Province, Malik Naveed Khan, said the death toll could rise.

'So far, according to my information, it was a security vehicle and an IED (improvised explosive device) was planted under the bridge,' Mr Khan said.

Police said that some of the victims appeared to have been civilians travelling in other vehicles which were near the bus when the blast took place.

Pakistan Air Force officials said they were still getting information about the attack.

Investigators were seen gathering evidence and material from the blast site. Two damaged motorcycles and bicycles also lay at the scene along with scattered debris, a witness said.

Officials said the bomb appeared to have been detonated by remote control and not a suicide attack, but said the type of bomb was still being looked into.

Pakistani forces are currently engaged in major battles against Taliban militants in a tribal area near Peshawar, but it was not immediately clear if the attack was linked.

More than 150 people, most of them militants, have died in the clashes in the Bajaur tribal area over the past week. Air strikes killed 50 militants alone on Monday, military officials said.

The Taliban have in the past carried out bomb attacks, mainly suicide blasts, targeting security forces in revenge for military operations.

The last major attack in Pakistan was a suicide bombing on July 6 at an Islamist rally to mark the anniversary of an army raid on the radical Red Mosque in Islamabad. That blast killed 19 people.

A wave of suicide attacks over the past year and a half has killed more than 1,000 people in Pakistan.

Pakistan's new government has been under major pressure from the United States to crack down on Taliban militants based near the border with Afghanistan.

The government has sparked concern in Washington, which regards it as a key ally in the 'war on terror', by entering peace talks with the militants after winning February elections. -- AFP