NATO said troops going to the crash site in southern Helmand province were ambushed by enemy fighters, and the unit called in an airstrike "to eliminate the enemy threat." It did not say if the troops were from the U.S.-led coalition, NATO's force or the Afghan army. One civilian was injured by gunfire.

Maj. John Thomas, a spokesman for NATO's International Security Assistance Force, said there would be a full investigation into Wednesday's crash. "We will try to determine everything that happened and to fully investigate the site," he said.

A U.S. military official, who insisted on speaking anonymously because the crash was still under investigation, said initial reports suggested the helicopter was hit with a rocket-propelled grenade.

A purported Taliban spokesman, Qari Yousef Ahmadi, claimed in a phone call to The Associated Press that militants shot the helicopter down in the province, the world's largest opium poppy-growing region where combat has been heavy in recent months.

Ahmadi did not offer any proof for his claim, but he specified the helicopter crashed in the Kajaki district hours before NATO reported that information.

Kajaki is the site of a large U.S.-funded hydroelectric dam now being repaired so it can provide electricity to the southern city of Kandahar. British troops, who make up the bulk of the forces in Helmand province, have been engaged in fierce fighting around the dam protecting it.

The Chinook, a heavy transport helicopter with two rotors, can carry about 40 soldiers plus a small crew. The fact it was flying at night suggested the aircraft might have been carrying troops on a nighttime air assault.

The NATO force, which is responsible for a countrywide counterinsurgency campaign, has 37,000 soldiers, including about 14,000 Americans. There are 12,000 U.S. troops in the separate coalition, which trains the Afghan army and conducts Special Forces anti-terrorism operations.

Helicopter crashes in Afghanistan have been relatively rare. A Chinook crashed in February in the southern province of Zabul, killing eight U.S. personnel. Officials ruled out enemy fire as the cause.

In May 2006, another Chinook crashed attempting a nighttime landing on a small mountaintop in eastern Kunar province, killing 10 U.S. soldiers.

In 2005, a U.S. helicopter crashed in Kunar, after apparently being hit by an RPG, killing 16 Americans.

Also Thursday, Afghan and NATO forces battled suspected Taliban in Helmand's Sangin district. The Defense Ministry said "tens of enemies" were killed or wounded, but did not provide further detail.

In western Farah province, insurgents attacked the Pusht Rod district late Wednesday, and ensuing clashes with police left 10 militants dead and 15 wounded, the Interior Ministry said. Retreating insurgents left behind one body and one wounded militant.

Elsewhere, a senior military officer said the U.S. had "the Taliban on their heels" in Afghanistan.

Brig. Gen. Perry Wiggins, deputy director of operations for the Joint Chiefs of Staff, told Pentagon reporters that "the Taliban has suffered numerous losses and has suffered losses to their senior leadership over this past month" in southern and eastern Afghanistan. As a result, the Taliban has "reverted to asymmetric-type, small-scale, high-profile attacks," he said.

Mullah Dadullah, a one-legged veteran who orchestrated an intensifying campaign of suicide attacks and beheadings, was killed in an operation in southern Afghanistan this month. He had been considered the top Taliban field commander.

Still, Wiggins said that as the poppy harvest ends, more insurgents might join the battle.