By David Clarke

BAGHDAD, Sept 28 (Reuters) - A Blackwater security guard screamed at colleagues to "stop shooting" in an incident that left 11 Iraqi civilians dead, enraged the government and sparked reviews of security firms in Iraq, U.S. media said on Friday.

The Washington Post and The New York Times quoted unnamed U.S. officials saying they had been told at least one employee of the private American security firm pointed a gun at a fellow guard to try and curb the shooting in Baghdad on Sept. 16.

Blackwater, one of the biggest private security operators in Iraq, employing 1,000 people, has said its guards reacted lawfully to an attack on a U.S. convoy. It was not immediately available for comment on Friday's media reports.

Citing a two-page U.S. embassy report, The Washington Post described an afternoon of mayhem including a car bomb, a shootout at a crowded junction and a standoff between Iraqi soldiers and Blackwater guards, eventually ended by U.S. troops.

"We're not commenting on the substance of the investigation," a spokeswoman for the U.S. embassy said.

Iraq has called the incident a flagrant assault and there are now several separate investigations, including a joint Iraqi-U.S. commission looking at private security firms used to protect U.S. government staff in Iraq.

Iraq says there are more than 180 mainly U.S. and European security companies in Iraq, with estimates of the number of private contractors ranging from 25,000 to 48,000.

Under a 2004 rule, the firms are immune from Iraqi law.

"STOP SHOOTING"

Citing the two-page "first blush" embassy report on the Sept. 16 incident, based on Blackwater testimony immediately afterwards, The Washington Post said the events that led to the deadly shooting involved three Blackwater units.

A State Department official quoted by the paper said it was only an initial account and the details could change as the investigations progressed.

The report said two units escorted a U.S. official back to the heavily fortified Green Zone in central Baghdad "without incident" after a car bomb exploded near a compound in which the official was having a meeting.

A third team sent from the Green Zone to help evacuate the official then came under "small arms fire" from "multiple nearby locations" at a road junction in Baghdad's upscale Mansour neighbourhood, the report said.

This version contrasts with statements from Iraqi police and witnesses who said Blackwater guards were the only ones firing.

The U.S. official familiar with the investigation told The Washington Post some of those involved in the shooting said at least one guard pointed a weapon at his colleagues.

"Stop shooting -- those are the words that we're hearing were used," the official was quoted as saying.

The U.S. official quoted by The New York Times said the words "cease fire" were used by one or more Blackwater guards and at least one carried on shooting despite the calls.

This unit then returned to the Green Zone. One of the first Blackwater teams sent back to help was surrounded by Iraqi security forces at the same junction.

There was a standoff and U.S. troops came to mediate. They escorted the Blackwater guards back to the Green Zone without incident, the report said.

Deputy Secretary of State John Negroponte, a former ambassador to Iraq, said "something went tragically wrong on Sept. 16 and we are taking steps to address the matter".

Speaking on Thursday, he said Blackwater had conducted 1,873 operations outside the Green Zone up to Sept. 18 this year and fired weapons on 56 of those missions. He gave no details but said they were reviewed to ensure procedures were followed.

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