KABUL - A helicopter that crashed two days ago, killing 38 people in the worst single incident in 10 years of war in Afghanistan, was carrying elite troops sent to help comrades in a firefight when it was likely hit by a rocket fired by the Taliban, NATO forces said on Monday.

In the first official indication of a possible cause of the crash late on Friday, the NATO-led International Security Assistance Force (ISAF) said the CH-47 Chinook helicopter was fired on "by an insurgent rocket-propelled grenade while transporting the US service members and commandos to the scene of an ongoing engagement."

The operation had begun when ISAF troops searching for a Taliban leader in the Tangi Valley, surrounded by rugged mountains in central Maidan Wardak province about 80 km (50 miles) southwest of Kabul, were fired on by insurgents.

Several of the insurgents were killed before assistance was requested, ISAF said in a statement.

"As the insurgents continued to fire, the combined force on the ground requested additional forces to assist the operation," the ISAF statement said.

"Those additional personnel were in-bound to the scene when the CH-47 carrying them crashed, killing all on board."

Thirty US troops - some from the Navy's special forces SEAL Team 6, the unit that killed Osama bin Laden in May - seven Afghan commandos and an Afghan interpreter were killed.

None of the SEAL team members who died in the crash took part in the raid that killed bin Laden.  

The Taliban quickly claimed to have shot the helicopter down on Saturday, although Western officials had remained tight-lipped while the aircraft and the bodies of those on board were being recovered.

ISAF imposed a security crackdown on the area while that grim task was completed, although some residents have complained about some of the measures that have been taken.

"I can only advise them not to try to approach the site of the crash while the investigation is ongoing," senior ISAF spokesman Brigadier General Carsten Jacobsen said .    

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.

SECOND HELICOPTER CRASH

At least another seven ISAF troops were killed in a ghastly weekend for the coalition. Four were killed in two separate attacks on Sunday, including two French legionnaires.

Earlier on Monday another NATO helicopter crashed in Paktia province, a volatile area in Afghanistan's east, but there were no apparent casualties and it appeared there was no enemy activity in the area at the time, ISAF said.

Taliban spokesman Zabihullah Mujahid claimed in a text message sent to Reuters that the Islamist group had shot down the second helicopter, another Chinook, in the Zurmat district of Paktia, killing 33 American soldiers.

The spike in casualties -- at least 383 foreign troops have been killed so far this year, almost 50 of them in the first week of August -- comes at a time of growing unease about the increasingly unpopular and costly war.

US and NATO officials issued statements vowing to "stay the course" in Afghanistan after Friday's helicopter crash but the devastating death toll will likely raise more questions about how much longer foreign troops should stay.

Afghan President Hamid Karzai spoke with US President Barack Obama by telephone overnight and shared condolences over the Wardak crash, Karzai's palace said in a statement.

The deaths came barely two weeks after foreign troops began the first phase of a gradual process to hand security responsibility over to Afghan soldiers and police.

That process is due to end with the last foreign combat troops leaving at the end of 2014, but some US lawmakers are already questioning whether that timetable is fast enough.

Karzai has already said "enemies of Afghanistan" -- the Taliban and other insurgents -- want to disrupt the process.