The convoy, which was guarded by USPI, was travelling to the Zahri district in Helmand province.
The blast destroyed two vehicles belonging to the guards and a civilian minibus.
Jan Mohammad, a police officer, said: "The bomb was so strong that it ripped through the civilian minibus and several other vehicles."
Body parts and pieces of metal from the bomber's car were scattered about 100m from the site. The outer wall of a nearby mosque were also partially destroyed.
One witness, who refused to give his name, said: "We saw 12 dead bodies being dragged away. They were civilians and also Afghan employees of the company."
The attack came a day after a bomber killed a district chief and three of his children at the gate of their home.
There was no immediate claim of responsibility for this attack, but the Taliban has claimed responsibility for many blasts in the region.
The group mainly relies on suicide attacks and roadside bombs in its campaign against the US-backed Afghan government and Western troops based in the country.
At least 7,000 people have been killed in the past 19 months despite the presence of 50,000 soldiers led by Nato and the US-led coalition, as well as more than 100,000 Afghan forces.
Attacks occur most in southern and eastern areas, where the Taliban and al-Qaeda are most active.
It has hampered reconstruction projects and has forced dozens of aid groups to end their activities.