The bomb attack took place in the town of al-Khalis on Monday morning.
There was no immediate claim of responsibility for the blast but the Takfiri Daesh (ISIS/ISIL) terrorist group regularly attacks security forces and civilian areas.
The head of the Provincial Council, Omar Maan al-Korawi, said a car rigged with explosives was detonated near the entrance to the Department of Agriculture in the town.
A police officer at the scene told the Reuters news agency that most of the victims died inside their vehicles while waiting to enter the town.
The death toll is expected to rise due to the extent of critical injuries, hospital sources said.