A suicide bomber riding a motorcycle slammed into a bus carrying defence department employees on Thursday, killing at least six people, police said. The attack, the latest in a string of deadly bombings in Pakistan, happened in Rawalpindi, a garrison city and home to the headquarters of Pakistan's military.

City police chief Nasir Durrani said at least six people died in the blast and around 20 more were injured.

"Apparently it was a suicide attack, and the attacker was on a motorcycle," Durrani said, adding that the blast targeted a government bus.

A senior intelligence official told The Associated Press that the bus was carrying employees of the Pakistani military's weapons-making department.

An AP reporter at the scene saw pieces of a charred motorbike lying on the ground, as well as a damaged bus, car and van. Shattered glass and twisted metal littered the road.

One witness, Mohammed Amin, said the motorcycle sped toward the bus before it detonated. "I saw a motorcycle getting closer to the bus, and I thought it would be crushed under the bus, but suddenly it exploded," said Amin, who was standing nearby. "I saw flames and dust blowing up and people were crying for help."

Islamist militants have carried out scores of bombing across Pakistan in recent years often targeting government security forces. Rawalpindi lies about 7 miles (12 kilometers) from the capital, Islamabad.