The bomb was buried near a spot where the soldiers often park their vehicles, said police official Azhar Hussain. The bomb exploded shortly after they arrived.
The soldiers were members of Pakistan's Rangers, a paramilitary force entrusted with maintaining security in conflict areas throughout the country.
Although Karachi is said to be a safe haven for Islamist militants, much of the violence in Pakistan's largest city has been linked to political and ethnic rivalries.
No one has claimed responsibility for the attack, police said. Source