A US Sea Knight helicopter has gone down near Baghdad, killing all seven crewmembers and passengers on board, the US military has said.
The US military has lost dozens of helicopters over Iraq
It said the cause of the crash - the fifth such incident this year - was being investigated.
This comes three days after the US said ground fire apparently caused the four previous helicopter losses, which claimed 20 lives.
The US has said it is modifying tactics and mission procedures as a result.
A US military statement said the aircraft went down in western Anbar province while conducting routine operations.
Anbar province has long been a hotbed of insurgent activity, claiming more US lives than anywhere else in the country.
'Ball of fire'
In Baghdad, an Iraqi air force officer said the helicopter was downed by an anti-aircraft missile, the Associated Press news agency reported.
Witnesses said they saw the helicopter go down amid gunfire from the ground, about 30km (18 miles) north-west of Baghdad, AP said.
RECENT HELICOPTER LOSSES
7 Feb: CH-46 Sea Knight helicopter goes down north-west of Baghdad, killing seven
2 Feb: Apache helicopter goes down near US air base at Taji, killing two
28 Jan: Apache downed while supporting Iraqi troops near Najaf, killing two
23 Jan: OH-6A helicopter used by private US security firm Blackwater crashes in Baghdad after coming under "heavy gunfire"; four civilians killed
20 Jan: Black Hawk goes down near Baghdad, with loss of 12 lives
"The helicopter was flying and passed over us, then we heard the firing of a missile," Mohammad al-Janabi, a farmer, was quoted by AP as saying.
"The helicopter then turned into a ball of fire. It flew in a circle twice, then it went down."
A statement purported to be from an al-Qaeda-linked group called the Islamic State in Iraq, posted on a website used by Sunni militant groups, said it brought down the helicopter.
"By God's grace, the downing of a helicopter of the crusaders was accomplished on Wednesday morning near the Karamah in Anbar province, leading to its destruction and the deaths of all those on board," the statement said.
Its authenticity could not be independently verified.
Last month, 12 US soldiers died when a Black Hawk military helicopter crashed north-east of Baghdad, the deadliest such incident this year.
The US has lost more than 50 military helicopters in Iraq since May 2003.