The incident, the deadliest single attack on American-led coalition forces since their March invasion of Iraq, happened outside the flashpoint town of Fallujah.
Iraqi witnesses said the Chinook was hit by one of two surface-to-air missiles fired at it, but the military has not yet commented on the cause of the downing.
The chopper was one of two twin-rotor Chinooks flying almost 60 people from a US military base to Baghdad, from where they were due to fly abroad for leave.
The downing of the helicopter was just one of a number of fatal attacks carried out on Sunday:
A roadside bomb in Fallujah killed two US civilian contractors working for the US Army Corps of Engineers and wounded one other, a Corps spokesman said.
A US soldier was also killed in a separate bombing in Baghdad, while witnesses reported four more wounded in an attack on a US convoy near the scene of the chopper strike.
US Defence Secretary Donald Rumsfeld acknowledged the spiralling coalition losses did not make for a “pretty picture” but vowed to “take the war on terror to the terrorists”.
The defence secretary insisted the new losses would not deter President George W. Bush’s administration from continuing its global war on terror.
“The only thing to do is… to take the war on terror to the terrorists,” Mr Rumsfeld told NBC television.
“You can’t just hunker down and hope they don’t hit you again.”
Meanwhile, foreign ministers from Iraq’s six neighbours plus Egypt urged the coalition to speed up the handover of power to Iraqis after a two-day meeting in Damascus called to address the threat to the region’s stability posed by the worsening violence.