Kabul, Afghanistan ? A U.S. military helicopter crashed Sunday morning in a dry, mountainous region of southern Afghanistan, killing all five crew members aboard, officials said.

Military authorities said there was no indication that the twin-rotor CH-47 Chinook came under fire before going down south of the Deh Chopan district of Zabol province, but they were still investigating the cause of the crash.

The helicopter was returning from dropping off U.S. troops in the area in support of an ongoing mission in Deh Chopan, U.S. military officials said. The region has been the site of fierce fighting this year between insurgents loyal to the Taliban, the repressive Islamic militia that controlled most of Afghanistan for five years, until the U.S.-led invasion in late 2001, and U.S. and Afghan forces, who are trying to flush them from longtime hideouts.

Since spring, armed fighters in groups of 20 or more have engaged in several hours-long battles with U.S. and Afghan forces in Zabol. The guerrillas have also conducted roadside bombings and ambushes against civilians, local leaders and government officials who are seen as supportive of President Hamid Karzai, as part of a broader campaign that has claimed more than 1,000 lives across Afghanistan.

A purported spokesman for the Taliban, Abdul Latif Hakimi, said guerrillas shot down the helicopter with “a new type of weapon.” In the past, however, Hakimi has often given exaggerated or incorrect accounts.

Weather also did not appear to have been a factor in the incident, said Sgt. Marina Evans, a U.S. military spokeswoman.

Evans said U.S. military forces had secured the site of the crash and recovered all five bodies. The identities of the deceased – two pilots and three others – were not released because their families had not been notified.

The deaths brought the number of U.S. service members killed in Afghanistan this year to 82.

In June, 16 U.S. service members were killed in northeastern Konar province when their special operations Chinook was shot down, apparently by guerrillas firing a rocket-propelled grenade. In April, a Chinook smashed into a desert plain in eastern Ghazni province in the midst of a sandstorm, killing the 15 U.S. service members and three American civilians aboard.

Seventeen Spanish soldiers belonging to a NATO-led force operating in the north and west were killed in a helicopter accident last month near the western city of Herat.