Eight US troops have been killed and 14 wounded in a helicopter crash in south-eastern Afghanistan, US-led coalition forces say.
Twin-engined aircraft with two, three-bladed rotors
Used to move soldiers, weapons and supplies
Can carry 54 troops or 25,000 lbs (11,340 kg) of freight - more than its own weight
Crew of four
The Chinook came down after the pilot reported engine trouble.
The crash happened in Zabul province, bordering Pakistan. A coalition statement said the Taleban had been building up forces in the area.
Coalition and Nato forces have lost several helicopters in Afghanistan in the last few years, most in accidents.
Initial reports indicated that Sunday's crash was purely accidental.
"It was not enemy fire related," Col Tom Collins, of Nato's International Security Assistance Force (Isaf), was quoted by the Associated Press news agency as saying.
"The pilot was able to radio in that he was having engine problems. We're confident it was not due to enemy action."
However, a statement issued by the US-led coalition forces later on Sunday did not specify the reason for the crash.
It said the CH-47 helicopter had "a sudden, unexplained loss of power and control and crashed in eastern Afghanistan".
It also said that: "Recent reporting indicated a Taleban build up for operations against the coalition forces in the region."
There were 22 people on board, including the crew.
"Coalition forces strongly advise any Afghans in the area of the crash to stay away from the site for their own safety," the statement said.
A Taleban spokesman said the helicopter had been shot down. Similar previous claims have turned out to be unfounded.
In April, 2005, at least 16 people died, 13 of them US personnel, when a Chinook crashed - the worst such disaster suffered by US forces there since the 2001 invasion.
In July, 2005, a US Chinook helicopter which was sent to back up a ground unit was shot down in the eastern province of Konar.
All 16 soldiers on board were killed. The Taleban say they brought it down.
Nato and coalition forces in Afghanistan are bracing for an expected spring offensive from regrouping Taleban fighters.