Major-General William Caldwell, the US military spokesman, told reporters: "We have a CH-46 that is down," referring to the twin-rotor Sea Knight, the marine version of the Chinook.
"There is a quick reaction force on site. It would be inappropriate for me to talk about casualties," Caldwell said.
Rash of crashes
A Chinook has two crew members and can carry up to 55 passengers. They are used widely to transport troops and cargo around
The reports came five days after a US army helicopter crashed in a hail of gunfire north of Baghdad, police and witnesses said - the fourth helicopter lost in Iraq in a two-week span.
US command said two crew members were killed in that crash.
Those helicopters were shot down in four separate incidents in which 21