NATO command centre in Kabul confirmed that US warplanes had been operating in the area
At least eight Afghan policemen have been killed in a US airstrike in the southern province of Uruzgan, local officials said.
"The first airstrike killed one policeman," said Uruzgan highway police commander Rahimullah Khan.
"When other policemen came to help, they came under a second air strike, killing seven of them," he added.
A policeman who survived the Sunday attack, Mohammad Sediq, said Afghan forces had been "engaged in close fighting" with the Taliban when the bombing happened.
The head of Uruzgan's provincial council, Mohammad Karim Khadimzai, told the AFP news agency the policemen "were inside their checkpoint" at the time.
The NATO command centre in Kabul confirmed that US warplanes had been operating in the area.
NATO spokesman Charles Cleveland said: "US forces conducted two airstrikes against individuals firing on... our Afghan partners in Tarin Kot on 18 September.
"We don't have any further information on who those individuals might have been or why they were attacking (Afghan) forces.
"US, coalition, and Afghan forces have the right to self-defence, and in this case were responding to an immediate threat."
A remote province with a large opium industry, Uruzgan has been one of the main points of conflict during the Taliban insurgency.
While NATO's combat mission officially ended in December 2014, US forces were re-empowered in their battle against insurgents in July, with President Obama pledging to leave 8,400 American troops in the country until his term came to an end.
The new arrangement gave US-led NATO troops more leeway to support Afghan troops in airstrikes.
Afghan forces - backed by US air strikes - launched an offensive to flush out Taliban insurgents encircling Tarin Kot earlier this month.
Civilian and military casualties have caused considerable concern in Afghanistan.
In July last year, a US air strike killed up to 10 Afghan soldiers at an army checkpoint south of Kabul in Logar province.
UN statistics show the Taliban are responsible for most deaths.