The bomber struck during the morning rush-hour near the headquarters of the Pakistani military, destroying the minibus and damaging several other cars. Security officials cordoned off the area.

Pakistan has been hit by a spike in violence linked to its struggle against Taliban and Al-Qaeda militants, raising fears for security in the nuclear-armed Islamic nation ahead of key elections set for February 18.

'It was a suicide attack, it appears that a man on a motorcycle packed with explosives rammed the bus,' the officer in charge of the local police station, Basharat Abbasi, said at the scene.

An army statement said six people including security personnel and civilians 'embraced martyrdom' and 38 others were injured.

The bus contained personnel from a military medical school in the city, security officials said. Most were trainees but one officer was among the dead, they said.

The blast blew off the roof, windows and doors of the bus, leaving it a charred wreck. Troops covered remains with a white tent while military police ordered journalists to stay away from the scene.

Eyewitness Shiraz Khalid, a motor mechanic, said he was buying breakfast when he heard a huge blast and rushed to the scene.

'The bus was completely destroyed. I saw dozens of people lying injured and dead on the road, covered in blood - most were wearing army uniforms. One was a woman,' Mr Khalid said.

'We shifted at least three people to hospital before the army came.'

Series of attacks
Rawalpindi has experienced a series of attacks on security forces in recent months which have been attributed to Al-Qaeda and Taleban militants based in Pakistan's northwestern tribal regions bordering Afghanistan.

A gun and suicide bomb attack on a political rally in a Rawalpindi park claimed the life of opposition leader Benazir Bhutto on Dec 27 and forced the postponement of national elections.

All of the blasts have been blamed on an Al-Qaeda-linked tribal warlord, Baitullah Mehsud.

Fighting between Pakistani forces and militants in the tribal areas has claimed the lives of more than 300 people since the start of the year.

Militants fired rockets on Sunday at an army camp in Miranshah, the main town in the North Waziristan tribal region, but there were no casualties, the army said.

Police also defused a roadside bomb containing 16 kilogrammes of explosive on Monday at Budaber in northwestern Pakistan, bordering the tribal regions, state media reported.

A US missile fired by a pilotless drone killed a senior Al-Qaeda commander in North Waziristan last week. Seven soldiers were killed in a presumed revenge attack in the area on Friday. -- AFP