A U.S. helicopter suffered a "sudden, unexplained loss of power" and crashed Sunday in southeastern Afghanistan, killing eight American troops, the military said. Fourteen people on board survived.

A NATO spokesman denied the helicopter had been shot down, saying the pilot had radioed ahead to report engine problems. It was the deadliest U.S. crash in Afghanistan since last May.

The CH-47 Chinook helicopter was carrying 22 U.S. service members under overcast skies when there was a "sudden, unexplained loss of power and control and crashed," U.S. military spokesman Lt. Col. David Accetta said. The 14 survivors suffered injuries.

The helicopter crashed in the Shahjoi district of Zabul province, about 50 yards from the main highway between Kabul and Kandahar, and appeared to be destroyed and scattered in several pieces.

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"It was not enemy fire related," Col. Tom Collins, spokesman for NATO's International Security Assistance Force, said earlier. "The pilot was able to radio in that he was having engine problems. We're confident it was not due to enemy action."

Zabul provincial governor Dilber Jan Arman said it was possible that the "helicopter crash was due to bad weather." There were no immediate claims of responsibility for the crash.

U.S. and Afghan military blocked reporters from entering the crash site.

In July, an AH-64 Apache attack helicopter plunged to the ground shortly after taking off from Kandahar Air Field while responding to a reported rocket attack against the air base. One crew member was killed.

In May 2006, a CH-47 Chinook helicopter went down while 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.

The military relies heavily on helicopters for transport and operations because of Afghanistan's forbidding terrain and lack of passable roads. Dust and the high altitude of Afghan's mountains take a heavy toll on helicopter engines.

Thousands of U.S. forces are deployed in southeastern Afghanistan.

Separately, NATO-led forces in southern Afghanistan shot to death an Afghan man they mistook for a homicide bomber, the alliance said Sunday.

The man was crossing a road Saturday about seven miles west of Kandahar and appeared to be approaching a NATO convoy, NATO's International Security Assistance Force said in a statement.

"He appeared to be chanting and refused to heed warnings to stop," the ISAF statement said. "As he continued his approach, members of the convoy noted what appeared to be a device with wires protruding from it on his chest."

The man failed to stop after the troops fired two warning shots, so they fired on him, said Capt. Andre Salloum, an ISAF spokesman in Kandahar.

The troops subsequently searched the man and found that "he had twine, straps and other materials protruding from his jacket, which resembled wires, but there were no explosives," the ISAF statement said.

"Any loss of life is a tragic affair; however, ISAF forces take every step necessary to protect themselves when confronted with threats or suspicious behavior," Salloum said.

NATO-led troops shot to death another Afghan man on Saturday as he ran between vehicles of a military convoy stopped near Kandahar's military airfield.

The man, whom the troops also believed to be a homicide bomber, was taken to a military hospital for treatment but died of his wounds, ISAF said in a statement Saturday.

President Hamid Karzai has repeatedly called on U.S. and NATO-led troops to exercise extreme caution to prevent civilian casualties. Dozens of civilian deaths during operations by foreign troops have undermined his authority among Afghans.

Afghan troops, meanwhile, detained 11 suspected militants Saturday at a checkpoint in Sangin district of the volatile neighboring province, Helmand, said Defense Ministry spokesman Gen. Mohammad Zahir Azimi.

The men were traveling in two trucks and were carrying mortar tubes when they were stopped, Azimi said Sunday.

In an operation that ended early Sunday, British and Afghan troops attacked a major Taliban extremist headquarters south of Garmsir in Helmand, said an ISAF statement. The troops destroyed three major compounds and a tunnel complex linking them.

There were no British or Afghan casualties. It was not immediately clear if any Taliban were killed or arrested in the operation.

Southern Afghanistan is the center of the growing Taliban insurgency as well as the world's biggest opium poppy-producing area.

Meanwhile, Al Qaeda released a video showing a young man asking for forgiveness from family, friends and teachers before he purportedly carries out a homicide car bombing against foreign troops in Afghanistan.

The video also carries previously released comments from Ayman al-Zawahri, Al Qaeda's No. 2 leader, as a train of armed men are shown walking through mountains and while an explosion hits a military vehicle on a turn in a road.

In the video, the man, who does not identify himself, asks his parents to pray for their patience when they get word that he has been "martyred."

Last year, militant supporters of the resurgent Taliban stepped up attacks, targeting Afghan government and foreign security forces. According to the U.S. military, there were 139 homicide attacks during 2006, up from 27 in 2005.

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