Fourteen US soldiers have been killed when their helicopter crashed in one of the worst incidents of its kind in the four-year war in Iraq.

The UH-60 Black Hawk helicopter was carrying four crew members and 10 passengers when it went down during a nighttime operation, according to a statement.

The US military said initial indications showed the aircraft experienced a mechanical problem and there were no indications of hostile fire, but the cause was still under investigation.

The 14 soldiers killed were assigned to Task Force Lightning, the military said.

The crash was the worst since January 2005, when 31 service personnel were killed after Marine transport helicopter was downed.

MSH/BGH