Afghan President Hamid Karzai says 31 US soldiers and seven Afghan troops have been killed in a helicopter crash.
It is one of the worst incidents of its kind in the 10-year-old war in Afghanistan.
A statement from the presidential palace says the helicopter crashed in central Maidan Wardak province, just to the west of the capital, Kabul.
The Taliban claim to have shot down the troop-carrying helicopter.
NATO has confirmed a helicopter has crashed but gave no further details.