Two U.S. service members were killed Thursday in a helicopter crash in Helmand province, the third fatal chopper crash in the south of Afghanistan in less than two months.
Taliban forces claimed to have shot down the aircraft.
The NATO force said an investigation was under way and that hostile fire could not be ruled out.
Two NATO helicopters were lost during June — one shot down and one as a result of mechanical problems.
Both of those deadly crashes also took place in the Taliban heartland, where most of Western military casualties occur.
A coalition military offensive centered on Kandahar, the south's main city, is gathering momentum after months of delays, with fighting heating up in outlying districts where Taliban fighters have long been in control.
The NATO force is heavily dependent on helicopters for troop transport, resupply runs and combat missions, because many of the roads in Afghanistan are poorly maintained and the rough terrain makes ground travel extremely difficult.
But helicopters are vulnerable to malfunction in the harsh climate, and NATO says the Taliban supreme commander, Mullah Mohammed Omar, has ordered field commanders to try to procure more heavy weapons, some of which could be used to target aircraft.
Thursday's crash took place near Lashkar Gah, Helmand's provincial capital. Thousands of U.S. Marines and British troops are deployed in the area, which lies close to the town of Marja, the scene of a major offensive earlier this year.
Afghan and Western officials, meanwhile, reported the arrest of an insurgent leader who they said had plotted to attack a major international conference earlier this week. The man was captured Wednesday night near the capital, Kabul, in a raid by NATO and Afghan forces, the military announced Thursday.