Islamabad: A roadside bomb ripped through a Pakistan Navy bus on a busy main road in the southern port city of Karachi on Thursday, killing at least five persons and injuring 15 others in the third such attack in two days.

The bus was targeted near a naval facility on Shara-e-Faisal road, officials said.

Deputy Inspector General of Police Iftikhar Tarar said the attack was similar to the bombing of two Pakistan Navy buses on Tuesday that killed four persons and injured 56 others.

The bomb, rigged to a motorcycle and containing about three kilograms of explosives, was triggered by remote control as the bus passed by, Tarar told the media.

Explaining the possible motive of the attack, Tarar said security agencies are the "main target" of terrorists.

Two persons were killed and 15 injured by the blast, state-run PTV reported. At least three vehicles, including a bus, and a petrol pump were damaged by the explosion.

The attack occurred at the same spot where slain former premier Benazir Bhutto's motorcade was attacked by two suicide bombers shortly after she returned to Pakistan from self-exile in October 2007.

No group claimed responsibility for today's attack. The Pakistani Taliban had claimed responsibility for Tuesday's attacks on two navy buses.