A suicide bomber rammed a motorbike into a bus carrying army medical trainees in the Pakistani garrison city of Rawalpindi on Monday, killing at least four people, officials said.
The attacker struck during the morning rush hour outside the army's National Logistics Centre, near the headquarters of the Pakistani army, police said. Security personnel cordoned off the area.
"It was a suicide attack, it appears that a man on a motorcycle rammed the bus," the officer in charge of the local police station, Basharat Abbasi, told AFP at the scene.
"There are four to five people who have been killed and about 25 people have been injured."
Security officials said on condition of anonymity that the minibus contained personnel from a military medical school in the city, which is the heart of the Pakistani military establishment.
Television footage showed the mangled wreckage of the vehicle, which troops later covered with a white tent. Military police were ordering journalists to stay away from the scene.
"I heard a huge blast and immediately rushed to the site. There was a small army bus, it was badly damaged with all the doors and windows blown out," said an eyewitness who gave his name as Nadeem.
"While I was trying to see if there were dead and injured people inside the army immediately came in and ushered me away," he told AFP.
Another witness, Haji Shaukat Khan, said he was opening his tyre shop in the Royal Artillery Bazaar about 200 metres from the scene when he heard a "gigantic explosion".
"There was a big ball of fire and smoke. Some pellets from the bomb hit the wall of my shop and I dived down, because I was injured in the arm in another blast that happened at this spot last year," he said.
Rawalpindi has experienced a series of attacks on security forces in recent months which have been attributed to Al Qaeda and Taliban 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 December 27 and forced the postponement of national elections.
A suicide bomber killed seven people near President Pervez Musharraf's military office in the city on October 30. Musharraf has since given up his role as chief of the army.
Two suicide bombers also blew themselves up in the city on September 4 last year, killing 25 people. Most of the dead were in a bus taking intelligence officials to work.
All those 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.
A US missile fired by a pilotless drone killed a senior Al Qaeda commander in the tribal region of North Waziristan last week.
Seven soldiers were killed in a presumed revenge attack in the area on Friday. (AFP)
Suicide attack on Pakistan military bus kills four