LASHKAR GAH, Afghanistan (Reuters) - A Taliban infiltrator killed 16 members of a pro-government militia force in the insurgency ridden southern province of Helmand when he turned his gun on men who he had worked with for months, officials said on Monday.

The Afghan security agency had set up the militia to infiltrate the Taliban, a security official told Reuters, though a spokesman for Helmand’s governor was unable to identify the group.

“We know that a Taliban fighter killed 16 militiamen fighting alongside government forces, but who these forces belong to, we don’t know yet,” said the spokesman, Omar Zwak.

The Taliban claimed responsibility for the killings in the province’s Gereshk district on Saturday, saying two of its fighters were involved.

A security official, who declined to be identified, said the suspected gunman had worked with the militia for months and took arms and ammunition after killing the men.

Helmand is one of Afghanistan’s most violent provinces, and also a major source of opium, the narcotic used to make heroin.

The Taliban, fighting to re-establish strict Islamic rule in Afghanistan and drive out international forces backing the government in Kabul, control large stretches of the province.