At least three security personnel were killed and four others injured when a bomb hit a vehicle of security forces in Pakistan\'s southern port city of Karachi Friday morning, reported local Urdu TV channel Duniya.The attack took place early Friday morning when a vehicle of Rangers, a paramilitary force in the country, was hit by a roadside planted bomb while on a routine patrol in the Safoora Chorangi area of eastern Karachi.The bomb was detonated by a remote-controlled device when the vehicle was passing by, said a Ranger\'s officer Brigadier Waseem, adding that the blast occurred when the vehicle was about 40 feet away from the planted bomb.Three Rangers personnel including a driver were killed on the spot and the remaining four injured people have been shifted to a nearby military hospital, said local media, adding that some of the shops nearby the blast site were also partially damaged.Police have cordoned off the area and a search operation is under way. Naeem Baroka, Deputy Inspector General of eastern Karachi, said an estimated five to six kilograms of explosives were used in the bomb, which was hidden under a pile of clay by the roadside.So far no group has claimed the responsibility for the attack.Friday\'s incident is one of the most serious terrorist attacks targeting the security forces in the country over the last couple of weeks.Immediately following the incident, Pakistani Interior Minister Rehman Malik condemned the attack and ordered an investigation into the case.