The CH-53 helicopter came down at around 10:15 pm (1815 GMT) while supporting what the US military described as "security and stabilisation operations".

"No enemy fire was observed in the vicinity of the aircraft. The cause of the mishap is currently under investigation," said a statement.

Three other people were injured in the incident and are being treated at a military medical unit.

The deaths bring to 240 the number of US troops killed in non-hostile incidents in Iraq since the 2003 US-led invasion, aside from the 691 US soldiers killed in action, according to Pentagon statistics.

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