An Afghan policeman turned his rifle on his colleagues at a checkpoint in southern Helmand province, killing 11 of them before fleeing the scene, officials said yesterday.
A police source told the BBC that an infiltrator from the Taliban had allowed militants into the police station in the regional capital of Lashkar Gah last night.
Bloodied corpses of the policemen were strewn around the checkpoint, many of them shot from close range, witnesses said, in a setback for Afghan forces before what is expected to be another fierce spring fighting season.
According to Umar Zwak, spokesman for the provincial governor, the attack took place in the provincial capital city, Lashkargah.
The Taliban militants, using firearms and grenades, seized police weapons and ammunition during the attack on Tuesday. He then collected all their firearms and fled the scene.
Police said it was not clear who was behind the attack and no one immediately claimed responsibility for the killings.
But a Helmand police official made no mention of a Taliban infiltrator in his comments on the attack.
In a similar incident last September, two Afghan soldiers with suspected Taliban links killed at least 12 of their comrades as they slept in the volatile northern province of Kunduz.
The attacker was recently transferred to Lashkargah from the province's Musa Qala district which had briefly fallen to the Taliban a year ago.
British and US forces suffered their heaviest casualties of the war in the province in years of fighting following the removal of the Taliban in 2001.