US-led coalition troops have killed four Afghan policemen and wounded one other after mistaking them for militants in southern Afghanistan.
The incident happened on Thursday in Helmand province's Girishk district, a US military statement said.
Insurgents have been active in Helmand since the toppling of the Taleban regime by US-led forces in 2001.
More than 1,000 people have died in violence linked to militants in Afghanistan this year.
'Not in uniform'
The incident in Helmand happened when US troops spotted a vehicle carrying five armed men in an area where the soldiers were fighting militants.
"Coalition soldiers shot at the vehicle, killing four of the individuals and wounding the other," the US military statement said.
US military spokesman, Lt Col Jerry O' Hara, told journalists that the police officers were not in uniform and that the vehicle had tried to drive away quickly from the area.
"We are conducting an investigation. Of course, we regret this incident," Col O'Hara said.
A senior police official in Helmand, Haji Mohammed Rahim, said the friendly fire incident took place after Taleban militants attacked a convoy of US and Afghan troops, killing two soldiers.
Afghanistan has recently held parliamentary elections which passed off without major violence, despite Taleban attempts to disrupt them.