"The lions of the Islamic State in Iraq succeeded in hitting a Black Hawk helicopter," it said.

"Clashes then broke out with the Crusaders. They resulted in the total destruction of two American Humvee vehicles, with all those on board killed," said the statement, whose authenticity could not be confirmed.

A senior US defence official acknowledged in Washington earlier Monday that the UH-60 Black Hawk helicopter may have been shot down with a shoulder-fired missile.

Pentagon spokesmen said a preliminary investigation was still underway and had not yet determined what caused the helicopter to crash on Saturday northeast of Baghdad, killing all those on board.

The deadliest helicopter crash involving US soldiers dates back to November 13, 2003 when two Black Hawks collided in the Mosul region of northern Iraq killing 17 troops.

earlier related report
Iraqi troops crucial in US Iraq plan : US Army Secretary
Washington (AFP) Jan 22 - The fate of the latest US plan to stifle violence in Baghdad hinges heavily on Iraq's government honoring its promises, US Army Secretary Francis Harvey said Monday.

More than 3,000 paratroopers, an advanced guard of 17,500 soldiers due to funnel into the Iraqi capital under President George W. Bush's new strategy have already arrived, the US military said on Sunday.

But Harvey's comments were the latest in a string of warnings by US leaders that the heaviest burden for restoring security rests on the much criticized government of Prime Minister Nuri al-Maliki.

"We also know that this is not totally a military solution, we all know it is a political, economic and military solution, in combination," Harvey said at an appearance at the Heritage Foundation think-tank here.

"The duly-elected government of Iraq is on the lead on this, we are in a supporting role, there are a number of political actions that must be taken, there are a number of economic actions that must be taken."

"If they come to fruition, and the Iraqis come through with the brigades that they have promised ... then I think that maximizes the probability to provide the security needed."

"If you don't have a secure environment, you can't have political stability."

Under the US plan, Iraq will send three brigades into the violence-torn capital in addition to the US troop contingent.

But some analysts have expressed doubts that the Iraqi forces are up to the job of taking the lead in the operation designed to crack down on militias.

There are also questions about whether units made of up Shiite and Sunni soldiers will take action against their own sects and if Kurdish soldiers will fight to the hilt, when they have no direct interest in the outcome.

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