The incident took place Friday around 5 pm as Afghan security forces were clearing a village of Taliban elements, Salam Afghan, Helmand police spokesman, said.
"In the strike, 16 Afghan policemen were killed including two commanders. Two other policemen were wounded," he said.
Omar Zwak, Helmand provincial governor spokesman confirmed the strike and gave the same account.
According to a press release by the US forces in Afghanistan, the US Air Force was supporting an operation by afghan security forces in the Gereshk District in Helmand Province, scene of some of the heavest fighting in Afghanistan in recent months.
"We struck a compound that we believed had hostile forces in it," Salvin said, "but it turned out they were local security forces aligned with afghan government forces."
An investigation will be carried out into the operation.
The number of American airstrikes in Afghanistan has increased greatly in 2017 compared to previous years. Half-yearly figures released on Tuesday showed that pilots have dropped more ammunition during aerial operations until end of June than in the whole of 2016.
Territorial gains by the Taliban and exhaustion among afghan ground troops has meant that military leaders have turned to airstrikes as a last resort to push back the Taliban.
The strikes have caused also civilian casualties to mount. This week, the UN posted half-yearly figures that showed the number of civilians killed or injured by airstrikes in 2017 to have risen by 43 per cent from the previous year.
American airstrikes were responsible for 37 per cent of victims, the afghan air force for 48 per cent.
US troops take part in a medevac exercise in Helmand province, Afghanistan July 6, 2017
A misdirected US airstrike on Friday resulted in the deaths of at least 16 members of the afghan security forces, the spokesman for the US military in Afghanistan confirmed.
