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.”