Nato has launched an investigation into the deaths of five US soldiers in southern Afghanistan as local officials blamed a "friendly fire" error by a coalition air strike.
Friendly fire incidents have been rare in Afghanistan in recent years, though five Afghan soldiers were killed in a Nato air strike in the eastern province of Logar in March.
"Five American troops were killed yesterday during a security operation in southern Afghanistan," Pentagon spokesman Rear Admiral John Kirby said in Washington. "Investigators are looking into the likelihood that friendly fire was the cause."
Helicopters were called in by US soldiers when they came under attack by the Taliban after a day's heavy fighting in Zabul province, east of Kandahar, Afghan officials said, but a bomb hit the wrong target.
"We had launched a clearance operation in an area with a high security threat,"said Zabul provincial police chief General Ghulam Sakhi Rughlewanai. "When it was over and we were returning to base, the enemy opened fire and [the US troops] asked for air support. The helicopters made a mistake and targeted their own people," he added.
Afghanistan is braced for Saturday's run-off presidential election - which the Taliban have vowed to target - and Monday night's joint US-Afghan operation was tasked with boosting security ahead of polling day.
Monday's deaths were the worst single incident for the Nato force since five British soldiers were killed in a helicopter crash on April 26.
Nato forces have officially stepped back from frontline fighting to focus on training Afghan soldiers, and death rates have dropped dramatically. But some troops still go out on operations to help Afghan soldiers in areas where they have weaknesses. The US-led force is winding down operations in Afghanistan after more than a decade of fighting Taliban insurgents.
All of the 50,000 remaining Nato combat troops are due to leave the country by the end of this year, although a small US deployment will remain until the end of 2016 if a long-delayed deal is signed between Washington and Kabul.
Southern and eastern Afghanistan are the most violent parts of the country as the Taliban wage a guerilla war against the Kabul government and remaining Nato troops.
Last month, US President Barack Obama outlined a strategy to end that country's longest war, saying that the 32,000-strong US deployment in Afghanistan would be scaled back to about 9,800 by the start of next year.
Those forces will be halved by the end of 2015 before eventually being reduced to a normal embassy presence with a security assistance component by the end of 2016.
Additional reporting by The Guardian, Associated Press