Witnesses reported seeing men armed with heavy machine guns open fire on the OH-58D Kiowa Warrior, a two-seat, single-engine helicopter, in al-Sukar district, north of Mosul, about 390km north of Baghdad.

The US military said on Friday: "Two pilots were killed on Friday when their OH-58D Kiowa helicopter went down in the city of Mosul... The aircrew members were recovered from the aircraft."

Lieutenant-General John Vines, the deputy commander of the US-led force in Iraq, confirmed that "the indicators are that it was due to hostile fire".

A Reuters cameraman in al-Sukar said US forces swiftly sealed off the area after the crash. The helicopter was completely destroyed and wreckage was strewn about. 

The US military said the aircraft had been conducting patrols with another OH-58D when it crashed. 

Friday's was the second US helicopter crash in Iraq in less than a week. A military UH-60 Black Hawk helicopter crashed on Saturday, killing all 12 on board.

The US military believes the Black Hawk may have been brought down by bad weather, but the cause is still under investigation.