The helicopter went down Sunday close to the US. military headquarters in Afghanistan at Bagram Air Base, about one hour's drive north of the capital, Kabul.

U.S. Central Command, in charge of operations in Afghanistan, says an investigation into the crash is under way.

It says the soldiers were involved Operation Mountain Resolve, a campaign in the northeastern provinces of Kunar and Nuristan aimed at rooting out militants waging an insurgency against the Afghan transitional government. The offensive began earlier this month.

In another incident just hours before the crash, two U.S. soldiers patrolling near the border with Pakistan were wounded when their vehicle struck an improvised explosive device.

Insurgents led by what is left of Afghanistan's hardline Taleban regime have increased attacks on Afghan and U.S. targets in recent weeks.

Vikram Parekh, senior Afghan analyst for the Brussels-based International Crisis Group, says this shows the Taleban are reorganizing since their fall from power in 2001. "I think it simply was a matter of time after re-assembling commanders, rebuilding funding networks and having eager troops to draw upon."

Other observers say the rise in attacks might be meant to destabilize Afghanistan ahead of its national convention, slated for December 10, to adopt a new constitution.