Coming on the heels of a spate of deadly helicopter crashes in Iraq, the incident is certain to raise new questions about US military engagements overseas at a time when President George W. Bush is beginning to campaign for reelection on the merits of his war on terror.

Although no formal announcement has been made, the downed helicopter is presumably a highly classified MH-53 Pave Low, one of the largest and most technologically advanced choppers in the world, used primarily in special operations.

"It was an MH-53 transport helicopter. The personnel on board were Army and Air Force," Major Mike Escudie, a spokesman for the US Central Command, told AFP.

The only other US helicopter of the MH-53 series, the Sea Dragon, belongs to the Navy and is used in minesweeping operations.

In a three-paragraph written statement, Central Command said the cause of the crash remained unknown and was under military investigation.

"The service members were supporting Operation Mountain Resolve," the command said without providing any further details.

But Escudie said the helicopter crashed as it was heading into the mountains from Bagram Air Base at about sunset and was destroyed on impact.

Earlier, a US CH-47 Chinook was shot down by a surface-to-air missile near Fallujah, killing 16 US soldiers.