A U.S. helicopter has crashed near Fallujah and some witnesses said a rocket-propelled grenade brought it down.

But an initial statement from the U.S. military described the crash as a "controlled" landing. Two people were on board the OH-58 observation helicopter belonging to 82nd Airborne Brigade but neither was injured.

The cause of the forced landing is unknown but small arms action by the enemy is suspected, the military said.

The Associated Press quoted a witness who said helicopters were flying in formation when one was hit by a grenade fired from the ground. The witness says the chopper went down immediately.

Reuters quoted one of its journalists at the scene as saying a helicopter had crashed.

Military officials could not immediately confirm the reports.