Thirty US troops, said to be mostly special forces, have been killed, reportedly when a Taliban rocket downed their helicopter in east Afghanistan.
Seven Afghan commandos and a civilian interpreter were also on the Chinook, officials say.
US sources say the special forces were from the Navy Seal unit which killed Osama Bin Laden, but are "unlikely" to be the same personnel.
This is the largest single US loss of life in the Afghan conflict.
The numbers of those killed have now been confirmed by the Nato-led mission in Afghanistan.
The Chinook went down in the early hours of Saturday in Wardak province, said a statement from President Hamid Karzai's office.
It was returning from an operation against the Taliban in which eight insurgents are believed to have been killed.
A senior official of President Barack Obama's administration said the helicopter was apparently shot down, Associated Press news agency reports.
An official with the Nato-led coalition in Afghanistan told the New York Times the helicopter was hit by a rocket-propelled grenade.
The BBC's Quentin Sommerville in Kabul says it is rare for the Taliban to shoot down aircraft.
The Taliban say they have modified their rocket-propelled grenades to improve their accuracy but that may not be true, our correspondent says.
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