Kabul, Afghanistan ? A U.S. military helicopter crashed before dawn Sunday in southern Afghanistan, killing eight American service members and injuring the remaining 14 aboard, military officials said.

The pilot had reported an unexplained loss of power shortly before the twin-rotor CH-47 Chinook transport helicopter plunged to the ground in Zabul province, which lies between the Afghan capital, Kabul, and the southern city of Kandahar.

It was the first U.S. military helicopter crash involving multiple fatalities since May, when 10 troops were killed in eastern Kunar province as the pilot tried to set down on a mountaintop in the dark. In July 2006, the crash of an Apache helicopter as it was taking off from the NATO base outside Kandahar killed one crew member.

The Sunday crash was also the deadliest single incident this year for U.S. and NATO forces in Afghanistan.

The U.S. military said there was no indication that the crash was caused by enemy fire, although the Taliban are known to be active in the area. The insurgents are expected to intensify their fight against allied troops as mountain snows melt and the weather improves.

Unlike in Iraq, where the downing of U.S. helicopters has emerged as a prime tactic of the insurgents, the Taliban have demonstrated little ability to menace allied aircraft. Only one U.S. helicopter is thought to have been brought down by hostile fire, in 2005.

Sixteen U.S. service members died in that crash, which apparently was caused by a rocket strike.

Rough terrain and often-turbulent weather pose substantial dangers to military aviation in Afghanistan. A mixture of rain and snow lashed parts of Zabul province overnight, and the provincial governor, Dilber Jan Arman, said through a spokesman that poor conditions might have been a factor.

“It was rainy and snowy,” said the spokesman, Gulab Shah Ali Khail, adding: “We don’t believe the crash was caused by enemy fire.”

Allied troops and Afghan soldiers cordoned off the scene of the crash, which scattered smoking wreckage within sight of the main Kabul-Kandahar highway. Passing vehicles were stopped and searched while troops combed the area.

The U.S. military declined to release any details about where the helicopter had taken off from, where it was bound or what its mission was.