A Taliban infiltrator has killed 16 pro-government militia fighters in Afghanistan's Helmand province.

The attacker had been fighting alongside the militia for months before turning his gun on them on Saturday night, at a checkpoint in the Gereshk district.

"We know that a Taliban fighter killed 16 militiamen fighting alongside government forces, but who these forces belong to, we don't know yet," a spokesman for Helmand's governor told Reuters news agency on Monday.

Local media said the Taliban claimed responsibility for the attack. The attacker managed to join the armed group after fleeing the checkpoint with weapons, TOLOnews reported.

Large swaths of Afghanistan's Helmand province - the country's opium production hub - are currently under control of the Afghan Taliban.

'Green on blue'

So-called green-on-blue attacks - where Afghan soldiers or police with Taliban sympathies turn their guns on colleagues - have been a major problem for Afghan and foreign forces.

Last June, three US soldiers were fatally shot and a fourth was wounded in eastern Afghanistan by an Afghan army soldier.

The killings have bred fierce mistrust between local and foreign forces, even as the rate of such incidents has dropped in recent years.

Such attacks are sometimes planned by the Taliban, but, mostly, they are caused by minor disagreements between the two sides.