NATO tried to determine on Sunday if Taliban insurgents had shot down a troop-carrying helicopter in Afghanistan, killing 38 in the largest loss of life suffered by foreign forces in a single incident in 10 years of war.
In a bloody two days for foreign forces, another four NATO troops were killed in two separate attacks on Sunday by insurgents in the country's violent east and south, the coalition said.
The majority of foreign troops serving in those regions are American, although some French troops are also based in volatile Kapisa province in the east.
The French president's office in Paris said two French soldiers were killed and five wounded when they were attacked by insurgents in Kapisa's remote Tagab valley. There was no immediate confirmation of the nationalities of the other two.
"The head of state expresses again France's determination to continue to work with the International Security Assistance Force (ISAF) to restore peace and stability in this country and contribute to its development," the president's office said in a statement.
Thirty U.S. soldiers -- some from the Navy's special forces SEAL Team 6, the unit that killed al Qaeda leader Osama bin Laden -- seven Afghans and an interpreter died in Friday night's crash, just two weeks after foreign troops began a security handover to Afghan forces.
The Taliban quickly claimed responsibility for bringing down the helicopter with a rocket-propelled grenade. Although it often exaggerates incidents involving foreign troops, a U.S. official in Washington said the helicopter was believed to have been shot down.
NATO-led ISAF 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.
Karzai called President Barack Obama and the two men expressed condolences for their countries' respective losses, the White House said.
"The two presidents reaffirmed their commitment to the mission in Afghanistan, which is critical to the security of both our countries, and agreed to stay in close contact," the White House said in a statement.
ISAF officials in Kabul remained tight-lipped on Sunday about possible causes of the crash and said the process of recovering the bodies from the site in a valley about 80 kilometres (50 miles) southwest of the capital was still ongoing.
The deadly crash comes at a time of growing unease in the United States and Europe about the increasingly unpopular and costly war. The last foreign combat troops are due to leave Afghanistan by the end of 2014 but some U.S. lawmakers are asking whether that timetable is fast enough.
The CH-47 Chinook crashed in central Maidan Wardak province in a hard-to-reach valley surrounded by rugged mountains.
Despite its proximity to Kabul, the area is one of the most dangerous in central Afghanistan, with fighters from the Taliban, the al Qaeda-linked Haqqani network and other militant groups all active.
"No words describe the sorrow we feel in the wake of this tragic loss," said General John Allen in a statement released overnight. Allen took over from General David Petraeus three weeks ago as commander of all foreign troops in Afghanistan.
The crash was the deadliest single incident for U.S. troops in Afghanistan since the Taliban was toppled by U.S.-backed Afghan forces in late 2001, ISAF said.
A U.S. official said some of the dead Americans were members of SEAL Team 6, but none of them had been part of the bin Laden raid in Pakistan in May.
Defense Secretary Leon Panetta said in a statement on Saturday the United States would "stay the course" to complete the mission in Afghanistan, a sentiment echoed by NATO Secretary General Anders Fogh Rasmussen.
The weekend's devastating death toll will likely raise more questions about the security transition and how much longer troops should stay.
"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," General Richard Dannatt, British army commander from 2006 to 2009, wrote in the Sunday Telegraph.
With doubts lingering about the cause of the crash, Karzai met his security advisers on Sunday to discuss what he called the "helicopter incident." He warned them to be on guard for more attempts by insurgents to derail the transition process.
"As there is the transition process going on, enemies of Afghanistan want to disrupt the national process by any means," Karzai said in a statement released by the presidential palace.
U.S. 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 the start of the war, with high levels of foreign troop deaths and record civilian casualties during the first six months of 2011.
Last year was the deadliest of the war for foreign troops in Afghanistan with 711 killed. The crash in Maidan Wardak and Sunday's deaths took the toll of foreign troops killed so far in 2011 to at least 383. More than two-thirds were American.