A Nato air strike has accidentally killed three Afghan policemen, threatening new friction between the force and a government struggling to find stability.
It was at least the fourth incident in a month in which coalition troops mistakenly killed civilians or friendly forces.
Afghan President Hamid Karzai has repeatedly complained about civilian deaths in Nato operations as the coalition tries to stamp out the persistent Taliban insurgency.
The strike came when US special forces were hunting Taliban fighters who had just carried out an attack in central Daykundi province.
The police killed in the strike were apparently mistaken for the insurgents.
Nato said a team on the ground called in air support after seeing "nine armed individuals setting up what appeared to be an ambush position". The men later turned out to be Afghan police, it said.
The coalition said it is investigating an apparent case of friendly fire.
On December 24, troops with the coalition killed two private security guards during a raid on what turned out to be a company compound after receiving intelligence that insurgents were planning an attack on the US embassy.
A day earlier, a Nato helicopter hunting for suspected militants opened fire on a convoy of cars heading to an event hosted by the head of a local council, killing a police officer and the brother of a former lawmaker. Earlier that week, a clash between Nato forces and insurgents left five civilians dead in Helmand.