A policeman was killed and a further 30 were detained in a joint operation between Afghan troops and U.S. forces last week against police suspected of supporting Taliban insurgents in embattled Helmand province, Afghan officials said on Sunday.

Reports of fighting between police and soldiers add to the upheaval in the southern province, long a stronghold of the insurgency, where the military has abandoned several outposts. The NATO-led coalition and the Afghan government are trying to overhaul security forces and reverse insurgent gains there.

The incident on Friday was in Sangin district, the scene of some of the heaviest fighting in Afghanistan’s long war, Helmand police chief Abdul Rahman Sarjang told Reuters.

“Army forces detained the police and took them to the military corps in Helmand,” Sarjang said. “An investigation is ongoing.”

The acting Sangin district police chief, Mohammad Nabi, was among the detainees, Sarjang said. Nabi was acting in place of the district’s official police chief, Mohammad Dawood, who is recovering from injuries.

Sarjang would not elaborate on the reason for the operation, but a senior Afghan army official in Helmand told Reuters that the army and U.S. advisers suspected the police of providing weapons and ammunition to the Taliban and that they had planned an eventual surrender to the insurgents.

“During our investigation we found some evidence they were helping the Taliban and we were afraid they may submit the district to the Taliban,” said the Afghan army officer, who asked not to be identified because he was not authorized to discuss the operation. “We launched a joint operation with Americans and detained all of them.”

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