The bombing happened during a US-backed Afghan operation to clear a village of Taliban insurgents in the Gereshk district on Friday, a police spokesman said.
It is believed the police were not in uniform – and this may have led to them being mistakenly identified as Taliban fighters, Helmand governor Hayatullah Hayat said.
Two commanders were understood to be among the dead – AND two other officers were also wounded in the airstrike, which was reportedly carried out by a drone.
NATO’s mission in Afghanistan said: “During a US-supported (Afghan security) operation, aerial fires resulted in the deaths of the friendly Afghan forces who were gathered in a compound.
“We would like to express our deepest condolences to the families affected by this unfortunate incident.”
The Pentagon has offered its condolences to the families of those killed and confirmed an investigation will take place.
Fighting in opium-rich Helmand has intensified recently, and US aircraft have conducted 52 airstrikes in the province over the past five days.
The Taliban controls much of the countryside in the province – threatening its biggest city Lashkar Gah – but the Afghan army has begun operations to push the insurgents back.
President Donald Trump’s administration has been considering sending more troops to Afghanistan as part of a new strategy for the region.
The US is the only foreign force in the coalition conducting airstrikes in Afghanistan.