Officials immediately ruled out enemy fire as a cause of the crash, which left charred wreckage of the twin-rotor Chinook scattered on a dusty, open plain in Zabul province, just 50 yards from the main Kabul-Kandahar highway.
This area of southern Afghanistan is one were Taliban fighters are known to be active and the Chinook workhorses are often used to transport troops around the rough terrain, reports CBS News foreign correspondent Mark Phillips. But U.S. military spokesmen won't say whether the soldiers who died today were on a combat mission.
There were no immediate claims of responsibility for any attack on the chopper, which went down under overcast skies in a region where Taliban militants are active.
It was the deadliest single incident this year for the 47,000 U.S.-led coalition and NATO forces in Afghanistan.
The helicopter was carrying 22 U.S. service members when it had a "sudden, unexplained loss of power and control and crashed," U.S. military spokesman Lt. Col. David Accetta told The Associated Press, adding the cause would be investigated.
"It was not enemy-fire related," said Col. Tom Collins, spokesman for NATO's International Security Assistance Force. "The pilot was able to radio in that he was having engine problems. We're confident it was not due to enemy action."
Seven U.S. Humvees and three Afghan military vehicles parked around the crash site. About 35 American soldiers and 15 Afghan army soldiers blocked reporters from entering the area. Afghan troops searched every passing vehicle and their passengers.
Zabul provincial Gov. Dilber Jan Arman said it was possible that the helicopter crash was due to bad weather.
A U.S. military statement said details of the crash or the helicopter's mission would not be released until "completion of recovery operations."
Afghanistan is a war being fought in Iraq's shadow, reports Phillips, but it is becoming deadlier and U.S. and NATO commanders here fear the Taliban have been quietly building up over this winter, preparing for a renewed offensive in the spring.
Zabul province has long been a hotbed for militant supporters of the former Taliban regime who have stepped up attacks over the past year.
In May 2006, another U.S. CH-47 Chinook crashed attempting a nighttime landing on a small mountaintop in eastern Kunar province, killing 10 U.S. soldiers.
In 2005, a U.S. helicopter crashed in Kunar, after apparently being hit by a rocket-propelled grenade, killing 16 American troops.
Another crash of a civilian helicopter last year in southeastern Khost province killed up to 16 people, including the wife and two daughters of a U.S. civilian worker.