Foreign forces in Afghanistan were investigating on Sunday the crash of a helicopter believed to have been shot down, killing 30 US soldiers, seven Afghans and an interpreter in the deadliest single incident for foreign troops in a decade of war.
The Taliban quickly claimed to have shot down the troop-carrying helicopter with a rocket-propelled grenade, although the Islamist militant group often exaggerates incidents involving foreign troops or Afghan government targets.
In Washington, a US official said the helicopter was believed to have been shot down. The Pentagon and the Nato-led International Security Assistance Force (ISAF) in Afghanistan said overnight the cause of the crash was being investigated.
The crash and its high death toll occurred two weeks after foreign forces started a security handover to Afghan troops and police -- to be completed by the end of 2014 -- and at a time of growing unease about the increasingly unpopular and costly war.
The Chinook crashed in central Maidan Wardak province, just west of the country's capital Kabul, on Friday night.
"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 the Navy's special forces Seal Team 6 -- the unit that killed al Qaeda leader Osama bin Laden in May in Pakistan, but that none of the dead had been part of the bin Laden raid.
The crash was the deadliest single incident for US troops in Afghanistan, ISAF said.
US defence 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.
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 figures kept by Reuters.