Five members of the US-led Nato force in Afghanistan died in a helicopter crash in the south of the country on Saturday, officials said, adding the cause of the incident was being investigated.
The crash was the largest single loss of life for the International Security Assistance Force (ISAF) since another Nato helicopter crashed in December after being hit in a Taliban insurgent attack.
"Five International Security Assistance Force service members died as a result of a helicopter crash in southern Afghanistan," the Nato force said. "ISAF is still in the process of reviewing the circumstances to determine more facts."
The statement did not name the province where the crash occurred and made no reference to whether any insurgents were active in the area.
But local officials told AFP the helicopter came down in volatile Kandahar and was not attacked by militants.
"A helicopter belonging to Nato troops has crashed in Takhta Pul, Kandahar province," Zia Durrani, the provincial police spokesman, said. "It was doing military exercises and crashed as a result of technical fault."
In line with Nato policy, the coalition deferred casualty identification to national authorities.
Six US troops were killed in the December attack when a Blackhawk chopper went down in the southern province of Zabul.
Immediately after that crash, US officers suggested the helicopter had come down due to a mechanical failure but that the crew may then have come under fire.
Officials later said that Taliban militants brought down the aircraft.
There was no immediate claim of responsibility from the insurgents after Saturday's crash.
In 2011, Taliban fighters shot at a US helicopter head-on from close range in what was the single deadliest incident for US and Nato forces in the Afghan war.
That attack killed 30 Americans as the chopper transported Navy SEAL commandos, along with other American and Afghan troops, to flush out a Taliban commander in Wardak province, near the capital Kabul.
Aircraft crashes have been a regular risk for the Nato mission in Afghanistan, with troops relying heavily on air transport to battle the Taliban insurgency across the south and east of the country.
Nato troop movements have fallen sharply over the last year as soldiers withdraw from the 13-year war.
From a peak of 150,000 in 2012, about 51,000 troops - 33,500 of them from the US - now remain in Afghanistan.
All Nato combat forces are due to pull out by December.
A small US force may remain from 2015 on a training and counter-terrorism operation if a long-delayed deal is signed between Kabul and Washington.
Last April, two US troops were killed when a helicopter crashed in the east of Afghanistan, while five US troops died in the southern province of Kandahar the month before when their helicopter came down during a heavy rainstorm.
Since 2001, 3,431 members of the US-led military mission have died in Afghanistan according to the independent Icasualties website.