"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."
It wasn't immediately clear how many people were on board or what type of helicopter it was. The coalition has launched a search and rescue operation for the helicopter and its occupants, said U.S. military spokesman Lt. Col. David Accetta.
"For the safety of the people on board, we will not release the number of people on board until rescue operations are complete," Accetta said. "Rescue operations are under way."
Accetta also declined to give details about the model of the helicopter and the exact location of the crash.
Zabul provincial governor Dilber Jan Arman said the helicopter fell in the Hassan Kariez area of Shahjoi district. He did not have any information about the number of casualties.
Arman said it was possible that the "helicopter crash was due to bad weather."
The military relies heavily on helicopters for transport and operations because of forbidding terrain and lack of roads. Dust and high altitude of the Afghan mountains take a heavy toll on the helicopter engines.
The incident Sunday was the first U.S. military helicopter crash since May 2006, when a CH-47 Chinook helicopter that attempted a nighttime landing on a small mountaintop crashed 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.
Thousands of U.S. forces are deployed in southeastern Afghanistan.