Foreign forces in Afghanistan on Sunday were investigating whether insurgents shot down a helicopter in what was the deadliest single incident for US troops in a decade of war.
Thirty US soldiers -- some from the Navy's special forces SEAL Team 6 that killed al Qaeda leader Osama bin Laden -- seven Afghans and an interpreter died in Friday night's crash which came just two weeks after foreign troops began a security handover to Afghan security forces.
The Taliban quickly claimed responsibility for bringing down the helicopter with a rocket-propelled grenade. Although it has in the past exaggerated incidents involving foreign troops, a US official in Washington said the helicopter was believed to have been shot down.
The NATO-led International Security Assistance Force (ISAF) in Afghanistan confirmed the death toll overnight, which was first announced by Afghan President Hamid Karzai, and said the cause of the crash was still being investigated.
The deadly crash comes at a time of growing unease about the increasingly unpopular and costly war. Foreign forces are due to complete their security handover to local troops and police by the end of 2014.
The Chinook crashed in central Maidan Wardak province, just west of the country's capital Kabul.
"No words describe the sorrow we feel in the wake of this tragic loss," General John Allen, who took over from General David Petraeus three weeks ago as ISAF commander, said in a statement released overnight.
"All of those killed in this operation were true heroes who had already given so much in the defense of freedom."
A US official said some of the dead Americans were members of SEAL Team 6. None of the dead had been part of the bin Laden raid in Pakistan in may.
The crash was the deadliest single incident for US troops in Afghanistan, ISAF said.
US Defense Secretary Leon Panetta said in a statement on Saturday that the United States would "stay the course" to complete the mission in Afghanistan, a sentiment echoed by NATO Secretary General Anders Fogh Rasmussen.
The crash will likely raise more questions about the security transition and how much longer troops should stay. All foreign combat troops are due to leave by the end of 2014, but some US lawmakers question whether that is fast enough.
"While acknowledging the immense personal tragedy of the loss of life in this helicopter disaster it is even more important to acknowledge that a greater tragedy would be to buckle under an understandable wave of emotion, and use it as a reason to withdraw now," wrote former British chief of the general staff General Lord Dannatt in The Sunday Telegraph.
US and other NATO commanders have claimed success in reversing a growing insurgency in the Taliban's southern heartland, although insurgents have demonstrated an ability to adapt their tactics and mount attacks in other areas.
But violence is at its worst in Afghanistan since US-backed Afghan forces toppled the Taliban government in late 2001, with high levels of foreign troop deaths, and record civilian casualties during the first six months of 2011.
Afghan President Hamid Karzai "shared his deep sorrow and sadness" with his US counterpart, Barack Obama, and the families of the victims, his palace said on Saturday.
Last year was the deadliest of the war for foreign troops in Afghanistan with 711 killed. The crash in Maidan Wardak means at least 375 foreign troops have been killed so far in 2011. More than two-thirds were American, according to independent monitor www.icasualties.com and Reuters figures.