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 said.
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 were 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 reported.
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 said it is rare for the Taliban to shoot down aircraft.
The Taliban have said they have modified their rocket-propelled grenades to improve their accuracy but that may not be true, our correspondent said.
“The president of the Islamic Republic of Afghanistan expresses his sympathy and deep condolences to US President Barack Obama and the family of the victims,” the statement from.
President Obama, too, issued a statement paying tribute to the Americans and Afghans who died in the crash.
“We will draw inspiration from their lives, and continue the work of securing our country and standing up for the values that they embodied. We also mourn the Afghans who died alongside our troops in pursuit of a more peaceful and hopeful future for their country,” the statement said.
Reports said 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 Bin Laden in Pakistan in May.
However, US officials have told both they BBC and AP they do not believe that any of those who took part in the Bin Laden operation were on the downed helicopter. (BBC)
