"This is a highly secure area controlled by the police forces," Hyder said.

'Confusion'

"We are getting reports that a vehicle carrying boxes of sweets drove into a highly secure area and was able to drive into the anti-terrorist compound, which is where the driver was told to wait outside while the person who brought his sweets went into the building," he said.

"We were told that the driver was not aware of what was being carried in his vehicle, so there is still some confusion as to whether this was a suicide attack or whether the person who had the trigger was able to make an escape after the blast.

"The only good news is that the number of security men deployed in the area during the time of the blast was minimal ... The attack was so powerful that it tore off a big chunk of that building."

The attack comes less than three weeks after more than 50 people were killed when a bomber detonated a lorry full of explosives at the Marriott hotel in the city.

Four children were among the 10 people killed in the roadside bombing in the northwest's Dir region.

Two police and four prisoners also died when the roadside bomb exploded under a prison vehicle, Sher Bahadur Khan, a government official, said.

Initial reports indicated a school bus was caught in the blast, but others said the children were walking near by.

'Heavy casualties'

Separately, five civilians, including women and children, were killed when a shell hit a house in the Matta district of Swat during clashes between troops and fighters, another security official told the AFP.

Also on Thursday, Pakistani fighter aircraft and helicopter gunships destroyed a suspected pro-Taliban facility in the northwest, killing 20 fighters, security officials said.

The air attack targeted a rebel stronghold in Piochar, a village in the Swat valley, a former tourist area once dubbed the "Switzerland of Pakistan", where troops have been battling fighters since last year.

In North Waziristan, a suspected US missile attack hit a house near the Afghan border killing at least nine people, including six foreign fighters, according to security officials.

Two missiles hit the building in the village of Tapi, an official based in the region told the AFP news agency on condition of anonymity. While another security official said that the attack targeted the house of Hafiz Sahar Gul, a local pro-Taliban commander.

The attack in Islamabad came as parliament entered into a closed-door question and answer session after a sensitive briefing by the country's military chiefs.

On Wednesday, Pakistan's freshly appointed intelligence chief addressed politicians on the domestic threat posed by al-Qaeda and the Taliban in Pakistan's tribal regions, in another closed-door session.