Afghan officials say a bomb exploded on Saturday in the western city of Herat, killing three civilians and wounding 16 others.
Police say the bomb was placed on a bicycle on the side of a road.
In a written statement, President Hamid Karzai condemned the attack.
On Friday, NATO's top commander, General John Craddock, said there has been a slight increase in suicide bombings and explosions in Afghanistan in recent weeks.
The supreme allied commander said that is the only indication so far of an upsurge in Taleban activity that had been predicted as the region's weather improves.
Also Friday, the United Nations said 25 Afghans living in Pakistan voluntarily returned home in the past week, marking the start of this year's repatriation efforts.
The UN says it has helped almost three million Afghans return home from Pakistan since 2002.
Agency officials say the Pakistani government is giving unregistered Afghan citizens until mid-April to repatriate.