An Afghan official says a roadside bomb has killed four civilians in southern Kandahar province.
The police chief of Kandahar's Shah Wali Kot district, Mohammad Bacha Khan, said the four were killed on Sunday when their car drove over the bomb.
The explosion occurred on a highway in the district.
The United Nations says roadside bombs planted by insurgents are the single biggest killer of civilians in Afghanistan, responsible for nearly one third of the 3,021 who died in 2011.
The U.N. attributed 77 percent of the deaths in 2011 to insurgent attacks and 14 percent to actions by international and Afghan troops. Nine percent of cases were classified as having an unknown cause.