A truck laden with explosives rammed into the wall of a paramilitary complex in Pakistan on Thursday and detonated, a military spokesman said.

The blast in Karachi injured 21 -- three critically -- and damaged the wall of a Rangers housing complex, said Maj. Bilal Farooq. No deaths were reported.

Farooq condemned the attack as a "cowardly act."

The truck had more than 220 pounds (100 kilograms) of explosive material, said Karachi police spokesman Javed Odho.

The Rangers are a paramilitary force in the Pakistani army often assigned to security detail in conflict regions.