Six Afghan policemen have been killed in an insider attack at their checkpoint in a volatile district of southern Helmand province.

Provincial officials told the BBC that a policeman killed six colleagues and fled with weapons and a vehicle.

The Taliban said the policeman had been recruited by them for the attack.

While many Nato soldiers have been killed in insider attacks, analysts believe most of the casualties occur within the ranks of Afghan forces.

Correspondents say reliable casualty figures for Afghan forces are hard to come by.

Earlier this week a policeman opened fire and killed seven colleagues in the Gereshk district of Helmand.

The latest attack had taken place in Helmand's restive Musa Qala district.

Officials say they are still investigating if the policeman had in fact been recruited by the Taliban.

Four of those killed were members of Afghanistan's locally based defence and militia force, and two of them were part of the national police force, officials said.

Nato combat troops are set to withdraw in 2014, leaving local forces to cope with the insurgency on their own.

Afghan forces are set to assume full security responsibility across the country next week, although foreign forces will continue to provide training and back-up.