American forces in Afghanistan suffered their biggest loss in a single incident on Saturday.
Thirty one US Navy Seals and seven Afghan commandos were killed when the helicopter crashed during an operation in east Afghanistan.
A Taliban rocket reportedly downed the helicopter.
A statement from President Hamid Karzai's office said the Chinook went down in Wardak province in the early hours of Saturday.
The BBC reports it was returning from an operation against the Taliban in which eight insurgents are believed to have been killed.
The New York Times reports the helicopter was hit by a rocket-propelled grenade.
However, the BBC says it is rare for the Taliban to shoot down aircraft.
An eyewitness told AFP that the helicopter had crashed as it was trying to take off after landing on the roof of a Taliban commander's home.
More than 20 of the US dead were Navy Seals.
A US military source has confirmed to the BBC that they were from Seal Team Six - the same unit which killed al Qaeda leader Osama bin Laden in Pakistan in May.
The US embassy in Kabul says operations to recover the helicopter are underway.
