His father, Gov. Ismail Khan of Herat, who was ambushed separately, escaped uninjured, Afghan state television said.
Sadiq was killed by gunmen as he sat in his car in Herat, a province bordering Iran, in which Khan has a large following and also commands a powerful militia.
The report said Khan's troops surrounded the home of a senior military commander and a militia barracks in Herat. Several people were reported killed and injured in fighting that erupted following the assassination.