Thirteen more were injured in the blast in Dihrawud in Uruzgan province .
The bomb could have gone off prematurely as the bomber waited for a military or police convoy to attack, said a deputy police commissioner.
On Wednesday, a UN report said the number of civilians killed in violence in 2009 was higher than in any year since the Taliban were ousted in 2001.
Civilian casualties rose by 14% in 2009 compared with 2008, the UN Mission in Afghanistan (Unama) reported.
