Officials at the Central Command's forward planning base here said there were a number of casualties among the soldiers on board but the crash was not believed to have been the result of Iraqi fire.

"The investigation on that is ongoing. We believe we do have some casualties as a result of that. We do not believe it was from enemy fire," Brigadier General Vincent Brooks told reporters.

A Defense Department official in Washington earlier said there were 11 people on board and "preliminary reports say seven were killed".

The Pentagon said the Blackhawk was apparently downed by small arms fire near the city of Karbala, south of Baghdad, and the four survivors were probably picked up in a rescue operation.

The Karbala region was the site of heavy fighting Wednesday and early Thursday as US troops moved to within just 15 kilometres (nine miles) of Baghdad.

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.

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