At least eight Afghans working with US forces have been killed in a Nato air strike in north-western Afghanistan, the defence ministry in Kabul says.
Nato confirms an air strike was ordered and is investigating whether it was the cause of the deaths on Friday.
It was launched as US and Afghan forces reportedly came under insurgent attack during search operations in Badghis province for two missing US soldiers.
Five Americans and 18 Afghans were also wounded on Friday.
Nato said the casualties occurred "during a joint operation that involved multiple engagements over several hours".
The Afghan defence ministry said a base housing coalition troops and Afghan security forces was mistakenly hit by the air strike.
Four Afghan soldiers, three police and another person believed to have been an interpreter were killed.
A spokesperson for the Nato force, US Navy Capt Jane Campbell, said: "We are saddened by the loss of life and injuries sustained during this very important mission."
Andrew North, BBC News, Kabul
It will be embarrassing for the US-led force if this does prove to be friendly-fire - coming just days after the controversy and criticism directed at Afghan security forces after five British troops were shot dead by a policemen they had been training.
One question that may be asked in this case is whether tighter rules on the use of what is known as 'close air support', laid down by the US commander General Stanley McChrystal, could have contributed.
He ordered the changes because of mounting anger among Afghans over civilian casualties caused by air strikes.
If Nato troops come under insurgent attack, in most cases they are now only supposed to call for an air strike if they are in danger of being over-run.
But this means insurgents are likely to be a lot closer by the time the order is made - therefore increasing the chance of aircraft thousands of feet above hitting the wrong target.
The Afghan deaths come days after an Afghan policeman killed five British troops in Helmand province, who the soldiers had been training.
The BBC's Andrew North in Kabul says that incident provoked fears about the possibility of Taliban infiltration into Afghanistan's armed forces.
Officials have said they believe the two US soldiers missing in western Afghanistan after failing to return from a routine resupply mission have drowned.
Afghan officials say the men, who were from 82nd Airborne Division, died two days ago in what appears to have been an accident.
Police said they died trying to recover supplies dropped by Nato aircraft which had fallen into a river.
A Taliban spokesman told Reuters news agency that they recovered the bodies in Badghis province.
Soldiers from more than 40 countries are taking part in Nato's force of nearly 110,000 troops in Afghanistan, two-thirds of them from the US.
Western Afghanistan has recently seen a rise in violence, with Taliban insurgents planting roadside bombs to maximise fatalities. This year more than 400 troops have been killed, most of them Americans.