An investigation is under way into reports that a US drone killed two American soldiers in Afghanistan. If confirmed, it would be a rare example of a “friendly fire” incident by the pilotless aircraft.

A military official said initial information indicated the soldiers had been conducting operations in Helmand province when they were killed by a Hellfire missile. It was unclear when the incident took place.

More than 1,500 US troops have been killed in Afghanistan since the war began nearly a decade ago.