A provincial spokesperson in the southern city of Lashkargah said the attacker detonated his explosives-packed car after trying to enter a police station.
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Spokesperson Omar Zhwak told AFP seven people had been killed, although a doctor at a local hospital said they had only received four bodies and 20 wounded.
"The attack killed seven civilians and wounded six, including two police. All the civilians were passers-by," said Zhwak.
"The attacker tried to enter the station, but hit a police vehicle outside the station and detonated."
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The bombing comes as Afghanistan has been battling a growing Taliban insurgency since Nato ended its combat mission in December after 13 years of war.
In January, at least 20 people were killed when a mortar hit a wedding party during nearby clashes between Afghan forces and insurgents.
Civilian casualties hit a new high last year in Afghanistan with about 10,000 non-combatants killed or wounded – 75% of them by the Taliban -- according to United Nations' figures.
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