Fifteen American soldiers were killed and 21 wounded when a US military transport helicopter crashed near Fallujah, Iraq, on Sunday morning.

The CNN's Jane Arraf, who was at the scene, said witnesses saw a shoulder-type missile strike the CH-47 Chinook helicopter just before it crashed into a field.

"They (coalition forces) are controlling traffic, with guns at the ready. They are saying it is still a volatile area," she said.

"The area is in the middle of farmland, and it would be extremely easy for somebody to hide here and launch a missile, which is what witnesses are saying."

The CNN quoted US military spokesman Colonel William Darley as saying the official cause of the crash had not been determined.

The chopper was carrying troops to Baghdad from an 82nd Airborne Division camp outside Fallujah.

Three other attacks also occurred on Sunday in which at least one US soldier died.

In Washington, Defence Secretary Donald Rumsfeld said the US would continue its war on global terror.

"Clearly it is a tragic day for Americans," Rumsfeld told ABC television.

"In a long hard war we are going to have tragic days."

Sunday's coalition toll is second only to March 23, the day 29 American troops died in combat.