A British patrol shot and killed a policeman Tuesday as troops left their base in an insurgency-wracked district in southern Afghanistan, officials said.

The policeman, armed and wearing civilian clothes, turned toward the patrol and was gunned down, mistaken for an insurgent, NATO said in a statement.

CountryWatch: Afghanistan

The troops gave him first aid and took him to the closest medical facility, where he died of his wounds, the statement said.

The cause of the shooting is unclear but will be investigated, said spokesman from Britain's Ministry of Defense speaking on condition of anonymity. He said the patrol was British.

The shooting happened in Musa Qala district in Helmand province, where four British troops have been killed this month.

Nearly 4,000 British troops are based in Helmand.