The passenger bus was traveling in Nimroz province on a main highway toward the capital, Kabul, when it struck the explosive around 7 a.m. local time, said Nazir Ahmad, a provincial government spokesman. Another 20 people were wounded, he said.
The explosion occurred around Delaram — a volatile area near the borders of Helmand and Farah provinces.
Meanwhile, NATO forces said a US service member was killed in an insurgent attack in southern Afghanistan on Tuesday, without giving further details.
July is already one of the deadliest months for US troops in the nearly nine-year Afghan war, with 59 service members killed so far. That's just shy of the 60 that died in June — currently the deadliest month for US forces.
US forces have pushed into southern Taleban strongholds in recent months and weeks in an attempt to squeeze insurgents out of the area where they have long functioned as a de-facto government. Along with the surge, attacks on military forces and Afghan supporters of the government have increased. Many civilians have also been killed or injured in incidents like Wednesday's bus bomb or caught up in the crossfire.
The NATO military coalition is also receiving heavy casualties overall. Altogether, 80 NATO troops have died so far in July. In June, 103 NATO forces were killed.
The rising death toll comes as US forces continue the search for a missing Navy sailor who was captured last week by Taleban forces when he and a colleague drove into an insurgent-held area of eastern Afghanistan. One of the sailors was killed in a firefight with militants and the Taleban has said they captured the other.
The sailors were instructors at a counterinsurgency school for Afghan security forces, according to senior military officials, who spoke on condition of anonymity because of the sensitivity of the case. The school was headquartered in Kabul and had classrooms outside the capital, but they were never assigned anywhere near where the body of the sailor was recovered, the officials said.
NATO officials were unable to say what the two service members were doing in such a dangerous part of eastern Afghanistan.
The Navy has identified the missing sailor as Petty Officer 3rd Class Jarod Newlove, a 25-year-old from the Seattle area. The Pentagon lists Newlove as "whereabouts unknown," and did not confirm that he was captured.
The soldier who was killed in the firefight was Petty Officer 2nd Class Justin McNeley — a 30-year-old father of two from Wheatridge, Colorado. NATO recovered his body on Sunday.
In central Uruzgan province, meanwhile, three Afghan soldiers were killed when their vehicle hit a roadside bomb Wednesday, said Gulab Khan, deputy provincial police chief.
