Six U.S. troops were killed and one wounded in a helicopter crash on Tuesday evening in the Shah Joy district of southern Zabul province.
U.S. military officials said that insurgents did not shoot down the helicopter, but they are investigating whether any of the U.S. troops were killed by gunfire from Taliban insurgents after the crash.
The Taliban has claimed responsibility for the crash while ISAF officials have announced the cause is still under investigation.
"Six International Security Assistance Force (ISAF) service members died following an aircraft crash in southern Afghanistan today," an ISAF statement said.
"The cause of the crash is under investigation, however initial reporting indicates there was no enemy activity in the area at the time of the crash."
The incident was the single biggest loss of life for the NATO mission in Afghanistan since seven Georgian soldiers died when a suicide bomber blew up a truck loaded with explosives outside a base in Helmand province in June.
The U.S. Defence officials in Washington told AFP the fatalities were US troops riding in a Blackhawk aircraft.
"I can confirm six Americans were killed," said one official, who spoke on condition of anonymity.
An investigation was underway to determine "the cause of engine failure," the official said.
Local Zabul officials have also confirmed the incident.
Before Tuesday's crash, there had been 149 NATO fatalities in Afghanistan this year, 119 of them US soldiers, according to the independent icasualties website.
NATO combat operations are due to end next year and Afghan forces will take over full security responsibility from the foreign troops.
There are currently 42,700 US troops deployed in Afghanistan as part of the NATO-led force that is due to withdraw by the end of 2014.