KANDAHAR, Afghanistan – An Afghan police officer and cook poisoned their colleagues at a checkpoint in an assault coordinated with insurgent fighters that left six dead in the country’s south, officials said Saturday.

It was the latest in a string of attacks from inside the Afghan army and police that are threatening to undermine both the partnership with international troops – which have been the target of many attacks – and the morale of Afghan forces, who have suffered equally heavy casualties from such strikes.

The police officer and the cook worked with outside insurgents in the assault, which hit police manning a checkpoint in the Gereshk district of Helmand province, the governor’s office said in a statement.