A policeman was killed and another 30 were detained during 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 at the moment."

The acting Sangin district police chief was among the detainees, Sarjang said.

Sarjang refused to confirm the reason for the operation but a senior Afghan army official in Helmand told Reuters the army and U.S. advisers suspected the police of providing weapons and ammunition to the Taliban and that they had planned to eventually 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 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."

The full extent of American involvement was not clear and a spokesman for the U.S. military in Kabul did not immediately respond to a request for comment.

The coalition recently deployed several hundred more troops to Helmand in a bid to increase security for the advisers helping Afghan forces. U.S. air strikes have also played an important role in trying to blunt Taliban offensives.

Almost 100 Afghan army officers were removed or reassigned in Helmand in recent months, and the army abandoned its outposts in several of the most hotly contested districts to redeploy elsewhere.