Authorities in southern Afghanistan say four Afghan police officers have been killed by their colleagues.

The local police chief, Mohammad Toryali, said the four were shot and killed at a checkpoint in the Gereshk district of Helmand province.

No one immediately claimed responsibility for the attack, the latest in a series of incidents in which Afghan security personnel or militants wearing police or army uniforms have opened fire on their Afghan and international counterparts.

So far this year, at least 55 coalition troops have been killed in such attacks. Several Afghan police officers and soldiers have also been killed by their colleagues.

The increase in such incidents comes as international troops continue to transfer security responsibility to Afghan forces and withdraw from the country ahead of a 2014 deadline.