The nation has been hit by a wave of bombings in recent weeks, following a military offensive against Taliban militants in the north west of the country.

"About 25 people were on board and, as the bus reached a square, a motorcyclist hit its fuel tank," city police chief Nasir Durrani told reporters.

"According to our reports, five to six people were martyred and 16 wounded."

TV footage showed pieces of a charred motorbike lying on the ground, as well as a damaged white bus, yellow car and white van. Shattered glass littered the road.

A senior police officer said the bus was carrying workers from a main nuclear facility, the Khan Research Laboratories.

A bomber attacked a bus carrying workers from the same facility in 2007.

The army went on the offensive in the Swat region two months ago after the Taliban seized a district 100km (60 miles) from capital Islamabad.

Rawalpindi, near Islamabad, is home to the headquarters of the Pakistani army and other state agencies.

The move raised alarm in Pakistan and among allies, who need its help to fight al Qaeda and tackle Afghanistan's insurgency.

Nearing the end of the offensive in Swat, the military has been attacking Taliban chief Baitullah Mehsud in his South Waziristan stronghold in mountains on the Afghan border.

The army says Mehsud, who carries a US reward of $5m and a Pakistani reward of 50 million rupees ($615,000), is behind 90% of militant attacks in Pakistan.