At least eight Afghan soldiers have been killed in a US air strike on an army checkpoint in Logar province south of Kabul, Afghan officials say.
They say two US helicopters attacked the checkpoint in broad daylight on Monday. Several troops were injured.
The army commander in the area told the BBC that the checkpoint was clearly flying an Afghan flag.
Logar is an unsettled area where much of the countryside is in the hands of the Taliban.
The BBC’s David Loyn in Kabul says that there is much confusion over the morning incident in the Baraki Barak district.
Our correspondent says that rescue efforts are being hampered by insurgents who are shooting at Afghan forces.
A spokesman for international forces said they were aware of an incident involving US forces and were investigating.
Analysis: BBC’s David Loyn in Kabul
There are still more than 13,000 international troops in Afghanistan – about half of them American. And more have remained for longer than President Obama originally ordered, after he acceded to military requests to slow down the withdrawal.
Since the Nato combat mission was wound up at the end of 2014, their principal mission is to “train, advise and assist” Afghan forces.
Most of the advice is at ministerial level and the top ranks of the armed forces, improving logistics and co-ordination.
The only active international fighting units are assisting Afghan special forces, and that is the mission that the helicopters believed to have been involved in the attack on Logar would have been engaged in.
There are air strikes, mostly from unmanned drones, somewhere in the country, every day.
Statistics recently emerged showing that more than 100 bombs were dropped in June – more than twice as many as any other month since combat operations ended.
Civilian and military deaths in coalition air strikes have been a contentious issue in Afghanistan since the US-led invasion of the country in 2001. They have provoked anger from the government and from many Afghan people.
The latest violence comes amid an increase in Taliban attacks on government and foreign targets during their summer offensive.
Last week Afghan Taliban leader Mullah Mohammed Omar backed peace talks with the government.
The two sides agreed to meet again in the coming weeks, drawing international praise.