Hostile fire has not been ruled out in the crash in Helmand province, said Lt. Commander Katie Kendrick, a spokeswoman for the military coalition.
Though helicopters more regularly go down because of mechanical issues in Afghanistan, some have been brought down by insurgent fire. In June, the Taliban shot down a helicopter in Helmand, killing four U.S. service members.
Taliban spokesman Qari Yousef Ahmadi claimed in a telephone call to The Associated Press that the insurgent group shot the chopper down.
Daoud Ahmadi, a spokesman for the Helmand provincial government, said the helicopter went down in the area of provincial capital Lashkar Gah.
The crash comes as violence is rising amid a surge of American troops into the south to try to squeeze the Taliban out of their strongholds in Helmand and neighboring Kandahar province. At least 50 U.S. troops have been killed in Afghanistan so far this month, putting July on track to be one of the deadliest months of the war for the United States.
Sixty U.S. service members were killed in June a record monthly death toll for the nearly nine-year war.
In the capital city, meanwhile, NATO and Afghan forces captured a suspected insurgent who had planned attacks against this week's international conference in Kabul.
The man was detained Wednesday night at a compound in the capital, the military coalition said in a statement. He is accused of involvement in plans to attack Tuesday's conference that was attended by delegates from 70 countries, including U.S. Secretary of State Hillary Rodham Clinton. NATO did not identify the detained man.
The plans were foiled by security forces and the conference passed without major incident, although rocket fire at Kabul airport forced the diversion of a plane carrying U.N. Secretary-General Ban Ki-moon and Swedish Foreign Minister Carl Bildt.
Afghan security forces virtually shut down Kabul for the conference, closing roads, setting up checkpoints and shuttering businesses and government offices.
Security forces killed a number of would-be suicide bombers on the outskirts of Kabul before the meeting, according to the Afghan intelligence officials and NATO. Then on Tuesday night, NATO forces said they detained another Taliban operative who had been in the final preparation stages for attacks against the conference.