Five Nato soldiers have been killed in the southern Afghan province of Zabul, with local officials saying they were mistakenly hit by a US air strike.
An Afghan army spokesman said the five troops, with an Afghan soldier and an interpreter, were killed by a bomb dropped from a coalition plane.
The International Security Assistance Force (Isaf) said the incident took place on Monday but gave no details.
Militants have stepped up attacks as foreign combat troops leave this year.
The Isaf force currently has soldiers from 50 contributing nations in Afghanistan.
Most troops stationed in the south are American, but Nato has not confirmed the nationality of these latest casualties.
The governor of Zabul province told the BBC there was a joint operation involving Afghan and international troops.
The incident happened in Arghandab district, a place hotly contested between the Taliban and international forces for some years, the BBC's David Loyn in Kabul reports.
Southern Afghanistan is the heartland of the Taliban movement and insurgents frequently attack security forces in the region.
There have been more than 30 Nato forces killed this year in Afghanistan - the latest incident is the deadliest so far in 2014.
Afghanistan is set to hold a run-off round of voting in its presidential election on Saturday.
Insurgents - who vowed to disrupt campaigning and voting - attacked the campaign rally of one of the contenders last week.