A group affiliated with al-Qaida in Iraq has claimed responsibility for the crash of a U.S. Army helicopter that killed its two crewmembers.

The crash happened Friday north of Baghdad. Witnesses said a pair of helicopters came under ground fire before one crashed. The other aircraft continued flying.

Four helicopters have crashed during the past two weeks. While official investigations are not yet complete, hostile fire is a possible cause of each crash.

At the Pentagon, the chairman of the joint chiefs of staff, General Peter Pace, said ground fire against U.S. helicopters in Iraq has been more effective in recent weeks.

In another development, the U.S. military said coalition forces killed 18 insurgents in fighting in the western city of Ramadi.

The military said insurgents opened fire on troops Thursday and Friday in the Sunni insurgent stronghold. It says coalition troops used "precision guided munitions" to halt the attacks.

The U.S. military also announced that two American troops died Thursday from wounds suffered in enemy action in al-Anbar province.

Some information for this report provided by AP.