A US helicopter crash in eastern Afghanistan has killed 31 US special forces and seven Afghan soldiers, President Hamid Karzai's office says.
The helicopter was taking the personnel back to their base after an operation.
It is not clear how the crash happened but the Taliban said they shot the helicopter down.
The incident is believed to be the biggest single loss of life for US forces in Afghanistan since operations began in 2001.
The helicopter went down overnight in Wardak province, the statement from President Karzai's office said.
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 said.
The Nato-led International Security Assistance Force has confirmed the helicopter crash but has not released details of casualties or the cause.
Nato said it was mounting an operation to recover the helicopter and find out why it crashed. It said there had been "enemy activity in the area" it went down.
A Taliban spokesman said insurgents brought down the helicopter with a rocket after US and Afghan troops attacked a house in the Sayd Abad district of Wardak where insurgents were meeting late on Friday, Associated Press said.
Sayd Abad, near the province of Kabul, is known to have a strong Taliban presence.
A Wardak government spokesman quoted by AFP news agency agreed with this, saying the helicopter was hit as it was taking off.
It is believed the helicopter was a Chinook, commonly used by coalition forces in Afghanistan to ferry troops in and out of combat.
There are currently about 140,000 foreign troops - about 100,000 of them American - in Afghanistan, fighting the Taliban insurgency and training local troops to take over security.
All foreign combat forces are due to leave Afghanistan by the end of 2014 and some troop withdrawals have already taken place.
Nato has begun the process of handing over control of security in some areas to local forces, with Bamiyan becoming the first province to pass to Afghan control in mid-July.
An increase in US troop numbers last year has had some success combatting the Taliban in the south of Afghanistan, but attacks in the north, which was previously relatively quiet, have picked up in recent months.