The deadly “friendly fire” incident took place in Afghanistan’s southern Helmand province on Thursday evening.
Afghan security forces called in US air support after taking heavy fire from Taliban fighters outside the province’s capital Lashkar Gah.
The US military said they targeted the area after being told that it was clear of allied units.
“Unfortunately, they were not and a tragic accident resulted,” said spokesman Colonel Dave Butler. “Afghan security forces as well as Taliban fighters were killed in the strikes.
“We’re examining the miscommunication to ensure it is not repeated. We regret this tragic loss of life of our partners and are committed to improvement every day with every mission.”
Another 14 Afghan police officers were injured in the strikes.
The police initially came under fire while attempting to take down a Taliban flag from a nearby water tower, according a report in the US military newspaper Stars and Stripes.
After failing to drive back the insurgents, the police called in the airstrikes.
Helmand’s governor, Mohammad Yasin, said Afghan authorities were investigating.
In March a similar ”miscommunication” resulted in the death of at least five Afghan soldiers during a US airstrike in neighbouring Uruzgan province.
The number of US airstrikes has risen in the past year, mostly in response to Afghan requests for assistance but also as part of an ongoing campaign against Isis militants in eastern provinces.
Taliban fighters continue to launch daily attacks on Afghan forces despite continued attempts to negotiate a peace deal to end the 18-year-long war.
On Saturday morning two people, including a child, were killed in a bomb blast near a market in the western province of Herat.
Fourteen others, including a district administrative chief, were injured in the explosion.
No one immediately claimed responsibility for the attack but Taliban insurgents regularly target Afghan officials and security forces in the area.
Last year saw the highest number of civilian casualties in Afghanistan since records began to be kept in 2009.
Additional reporting by Associated Press