An explosion ripped through a police truck today to kill at least seven people in Pakistan's southwestern city of Quetta, police said.

The blast occurred when a Rapid Response Force truck carrying as many as 30 personnel.

The truck was carrying police recruits, four of whom were killed, on the city's eastern outskirts, Quetta Police Chief Abdur Razzaq Cheema said.

An emergency has been declared in the hospitals across the city.

"We can't say whether it is a suicide blast," he added. "Our teams are at the scene and investigating."

Television broadcast images of the burned-out remains of the vehicle.

-With inputs from Hamza Ameer