Six Afghan policemen have been killed after an officer and a cook turned on them in Helmand, officials say.
The cook had tried to poison the men at a checkpoint in Gereshk district on Friday, said officials.
He and another policeman then shot two of their colleagues dead. The other four were killed by insurgents who attacked the checkpoint.
It was the latest in a spate of attacks by Afghan security personnel against their local and foreign colleagues.
The policeman who attacked at the checkpoint has since been captured by security forces in Musa Kala district, and is under investigation, said the Helmand governor's office.
The Taliban have claimed the attack, saying they had despatched a "mujahid who had infiltrated and penetrated the police ranks" before killing eight policemen, spokesman Qari Yousuf Ahmadi was reported as telling Reuters news agency.
The growing number of insider attacks on Afghan security targets or coalition forces has raised questions about both recruitment procedures and the trustworthiness of the tribal leaders who endorse each recruit, analysts say.
Military commanders call these rogue attacks "green-on green" or "green-on-blue" - green for Afghan forces and blue for the coalition.
Dozens of local police and Afghan soldiers and some 50 foreign troops have died this year in such attacks, prompting Nato troops to arm themselves on all patrols and the introduction of more stringent vetting procedures by the Afghan security services.
Official Nato analysis says about a quarter of rogue attacks are the result of Taliban infiltration or influence, with the rest being due to individual grudges or disputes that get out of hand.
The killings come at a sensitive time for Afghanistan, says BBC defence correspondent Caroline Wyatt, as Nato forces continue to hand over responsibility for security to the Afghans ahead of the withdrawal of Western combat forces by the end of 2014.