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.
Neither the US nor Nato have confirmed the cause, but witnesses, officials and the Taliban say it was shot down.
The incident is believed to be the biggest single loss of life for US forces in Afghanistan since operations began in 2001.
The Chinook 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.
Nato's worst Afghan moments
6 April 2005 - Chinook crash in Ghazni province kills 15 US soldiers and three civilian contractors
28 June 2005 - 16 US troops killed when Taliban bring down Chinook in Kunar province
16 August 2005 - 17 Spanish soldiers die when Cougar helicopter crashes near Herat
5 May 2006 - 10 US soldiers die after Chinook crashes east of Kabul
2 Sept 2006 - 14 UK personnel killed when RAF Nimrod explodes following mid-air refuelling
18 August 2008 - 10 French soldiers killed in Taliban ambush east of Kabul
6 August 2011 - 31 US special forces and seven Afghan soldiers killed in Chinook crash
Source: BBC and news agencies