"When I came to, I got out of the bus and saw that the bus was totally wrecked"
Lal Jan, survivor
"Civilians are not our target," he said.
Bismullah Khan, the police chief in the district of Maiwand, said that some of the 39 injured had been taken to a nearby Nato base for treatment.
Others were taken to the main hospital in Kandahar.
Lal Jan, a survivor, told The Associated Press news agency: "An explosion hit the bus. I don't know what happened. When I came to, I got out of the bus and saw that the bus was totally wrecked."
Ousted from power in a US-led invasion in 2001, the Taliban largely relies on roadside bombs and suicide attacks in its campaign against foreign and Afghan forces.
More than 1,500 civilians have been killed by the battle for control of Afghanistan so far this year, the United Nations said last week.
It said that 68 per cent of the civilian killings were a result of Taliban attacks, while 23 per cent were caused by Afghan and foreign troops.
Homemade bombs have become by far the deadliest weapon used by the Taliban against Western and Afghan government forces. The devices kill many more civilians than soldiers.
Zeina Khodr, Al Jazeera's correspondent in Kabul, the capital, said: "Civilians are increasingly being caught in this ongoing conflict.
"This is really the main weapon of choice for the Taliban - the roadside bombings, the IEDs - they have caused a high number of casualties among the international forces.