Maj. Gen. William Caldwell, spokesman for the multinational forces in Iraq, said a CH-46 helicopter went down about 20 miles northwest of Baghdad. Associated Press/Wathiq Khuzaie

A U.S. military statement gave no reason for the crash of the CH-46 Sea Knight, which went down near Fallujah in Anbar province, about 20 miles from Baghdad. However, at the Pentagon, three Marine Corps officials said the troop-transport helicopter was in flames when it went down, with the pilot appearing to attempt a hasty landing but losing control as the aircraft descended.

They said witnesses in nearby Marine aircraft saw the flames but saw no sign that it involved hostile fire.

An Iraqi air force officer, however, said the helicopter was downed by an anti-aircraft missile. He spoke on condition of anonymity because he was not authorized to release information.

An Iraqi farmer who lives about a half mile from the crash site said he heard a missile fired moments before the crash, which took place in an insurgent-infested region.

"The helicopter was flying and passed over us, then we heard the firing of a missile," the farmer, Mohammed al-Janabi, said. "The helicopter then turned into a ball of fire. It flew in a circle twice and then went down."

Associated Press Television video showed the flaming wreckage lying in a field in front of a cluster of mud homes. A dense plume of black smoke rose over the remains. The Marine officials suspected the fire was caused by a mechanical problem, the officials said. They spoke on condition of anonymity because the investigation was ongoing.

In a statement posted on an extremist Web site, an al-Qaida-linked group, the Islamic State in Iraq, claimed it shot down the helicopter, which it described as a Chinook — an Army helicopter which resembles a Sea Knight.

But critics have long urged the military to replace the CH-46, which was introduced in 1964 at the start of the Vietnam War. In 2001, retired Col. Frank Jensen wrote in Defense News that the Marines should replace the CH-46 but cannot because of budget limitations.

Regardless of the cause, the latest crash adds urgency to a U.S. military review of flight operations in Iraq, including whether insurgents have perfected skills in attacking U.S. planes.

The latest crash occurred five days after a U.S. Army Apache helicopter went down in a hail of gunfire north of Baghdad. Three other helicopters — two from the Army and one operated by an American security firm — also have crashed since Jan. 20. A total of 20 Americans were killed in those four crashes.

The military has said the four aircraft were all believed to have been shot down, raising new questions about whether Iraqi extremists are using more sophisticated weapons or whether U.S. tactics need changing.