Dozens of soldiers have died after the Taliban shot down a Nato helicopter - in the deadliest single attack on foreign troops since the Afghanistan war began in 2001.
Earlier reports said more than 20 Navy Seals from the unit that killed Osama bin Laden were among those killed in a helicopter crash in Afghanistan.
The operators from Seal Team Six were being flown by a crew of the 160th Special Operations Aviation Regiment, US officials officials.
One source says the team was thought to include 22 Seals, three Air Force air controllers, seven Afghan Army troops, a dog and his handler, and a civilian interpreter, plus the helicopter crew.
Afghan officials at president Hamid Karzai 's office said 31 US special forces and seven local soldiers died in the attack on the Chinook.
A senior US official has confirmed the helicopter was apparently shot down by insurgents.
Eyewitnesses said the Chinook was struck during a firefight and the Taliban claimed responsibility for the incident.
It took place during an anti-Taliban operation in an insurgent-infested district of the eastern province of Wardak, just southwest of the Afghan capital Kabul.
The Afghan statement said: "The president of the Islamic Republic of Afghanistan Hamid Karzai expressed condolences over a Nato helicopter crash and the deaths of 31 members of US special forces."
It added that seven Afghans were also killed in the crash, who the country's defence ministry said were also members of the special forces.
The strike was by far the worst to hit foreign troops in the near decade-long war. The previous worst saw 16 American soldiers killed in 2005 when a Taliban rocket hit their Chinook in the eastern province of Kunar.
Mohammad Saber said he saw the helicopter plummet to the ground during an operation in his village.
He said: "We were at home. We saw one of the helicopters land on the roof of a house of a Taliban commander, then shooting started.
"The helicopter later took off but soon after taking off it went down and crashed. There were other helicopters flying as well."
Wardak provincial spokesman Shahidullah Shahid said the crash came during a late-night operation against Taliban insurgents.
"We're not sure if this happened due to insurgent fire or not but there were some casualties of foreign troops due to the crash," he said.
Taliban spokesman Zabiullah Mujahid said it was responsible for shooting down the helicopter and acknowledged that eight insurgents had been killed.