A Defense Department official said, "There were 11 on board. Preliminary reports say seven were killed."

The four survivors were probably picked up in a rescue operation, the official added.

However, a report of the incident posted on Central Command's website said only six were aboard the UH-60A Blackhawk troop transporter, adding, "casualties have not been confirmed at this point...The cause of the crash is being investigated."

The Pentagon said the Blackhawk was apparently downed by small arms fire near the city of Kerbala, south of Baghdad.

Central Command said the helicopter was on "an operational mission" when it crashed.

The Kerbala region was the site of heavy fighting Wednesday and early Thursday as US troops moved to within just 50 kilometres (30 miles) of Baghdad, surging north in a major offensive that Pentagon officials said had left Iraq's key Republican Guard divisions in tatters.

Iraqi forces tried to counterattack but were repelled by coalition forces.

Helicopters have been at the center of several deadly incidents during the war in Iraq.

Nineteen US and British servicemen were killed in the first two days of the conflict in helicopter accidents in the Gulf region.

Three Americans were killed and one was injured when a UH-1 Huey helicopter went down last Sunday.

The Iraqis claim to have shot down several Apache attack helicopters.

SPACE.WIRE