David Accetta, a lieutenant colonel and coalition spokesman, said: "For the safety of the people on board we will not release any information about the the type of aircraft or the number of people on board until the rescue operations are complete."
Tom Collins, an International Security Assistance Force spokesman said: "It was not enemy fire related. The pilot was able to radio in that he was having engine problems. We're confident it was not due to enemy action."
The helicopter went down near a road in the Shahjoi district of the province of Zabul, according to residents there.
Dilber Jan Arman, Zabul's provincial governor, said the helicopter fell in the Hassan Kariez area of Shahjoi.
He did not have any information about the number of casualties.
The governor said it was possible that the "helicopter crash was due to bad weather.
Taliban fighters are active in the southern province on the border with Pakistan.
International forces have lost several helicopters in recent years but only one is believed to have been shot down.
That aircraft was hit by a rocket-propelled grenade as it was coming in to land during a clash.
The last aircraft to crash was in early December, when a civilian helicopter went down between the southern city of Kandahar and Uruzgan province. All eight people on board were killed.
Taliban fighters claimed to have downed the aircraft, a Russian MI25, using a rocket, but officially the cause of the crash has not been made public.