The chopper came down on Thursday during a "routine" mission near Falluja, a hotbed of anti-occupation attacks, said a US military spokesman, giving no details as to the cause of the crash.
But witnesses told our correspondent: "The warplane was hit by a missile and we rushed to the area to see a trail of smoke billowing up in the air from the burning chopper."
Two choppers landed by the crash site, while another two hovered overhead.
In the deadliest single attack on US forces since the Iraq invasion began in March, 17 soldiers were killed on 15 November when two Black Hawk helicopters collided above Mosul.
Meanwhile, a US Air Force C-5 cargo plane carrying 63 passengers and crew was apparently hit by a missile on Thursday as it took off from Baghdad airport, but landed safely, said a senior US defense official.
A surface-to-air missile hit the plane but it was able to land, said the official, speaking on condition of anonymity.
US helicopters down in Iraq since President George Bush declared major combat over on 1 May:
12 June - Apache shot down in western Iraq, crew unharmed. US military declines to say if it shot down.
25 Oct - Black Hawk downed by a rocket-propelled grenade
near Tikrit. One soldier wounded.
2 Nov - Chinook shot down near Falluja. Sixteen US troops
killed and 21 others on board injured.
7 Nov - Black Hawk comes down near Tikrit, probably hit by a rocket-propelled grenade. All six people aboard killed.
15 Nov - Two Black Hawks collide under fire in Mosul,
killing 17 soldiers. It remains the bloodiest single incident
for US forces since they invaded Iraq on March 20.
9 Dec - Kiowa crashes near Falluja after coming under fire.
Both crew walk away with slight injuries.
2 Jan, 2004 - OH-58 observation helicopter shot down in central Iraq, killing one pilot and injuring another.
8 Jan- Black Hawk on medical evacuation mission comes down
near Falluja, killing all eight people aboard.
An investigation was under way with experts sent from Europe, said the air force's Air Mobility Command.
"Initial reports indicate the incident is the result of hostile
action from the ground but the type of weapon and other details are unknown," the command said in a statement.
Earlier, the air force said the C-5 declared an inflight emergency "because of excessive engine vibrations in their number four engine."
There were no injuries reported among the 11 crewmembers and 52 US military personnel.
It would be the third time since May 1, when major combat operations were declared over, that a plane has been hit by a
surface-to-air missile while flying out of Baghdad international airport.
In December an Air Force C-17 cargo and troop transport plane was hit by a surface-to-air missile after takeoff from Baghdad with a crew of three and 13 passengers.
And in November, a DHL cargo plane was hit by a shoulder-fired SA-14 surface-to-air missile as it took off from Baghdad airport. DHL temporarily suspended flights into Iraq after the incident.
Elsewhere in the country, a Kurd was shot dead by unidentified attackers and two bombs were defused in the ethnically-tense northern Iraqi city of Kirkuk on Thursday, said police.
Kurds have been demonstrating
for autonomy over Kirkuk
US occupation troops defused bombs that were placed in front of the Jumhuriya Bank and the Kirkuk Technical School, said Colonel Khatab Abd Allah Araf of the city's civil emergency service.
He said five Russian Strella rockets were also found in an
office building in the city.
US military leaders also met with the heads of Arab, Kurd, Turkmen and Assyrian parties in the city that reflect the city's four main ethnic groups.
The parties were asked to limit the number of armed security
guards outside their offices to four or five, and not to stage any demonstrations in Kirkuk without prior approval from the US military, said a local police official.
Kirkuk, which Kurds want to incorporate into an autonomous
Kurdish region has been hit since the start of the year by deadly unrest between Kurds, Arabs and Turkmen.