The head of the army's aviation center has reported separately that insurgents are now using a new "aerial" improvised explosive device (IED) designed to shoot 15 meters (50 feet) into the air and explode under a low-flying helicopter, the weekly Defense News reported.

Lawrence DiRita, a Pentagon spokesman, said there was no reason to believe such a device was used to bring down any of the three helicopters that have crashed since January 8.

But he said investigators are trying to determine "is there a trend, is there a new threat out there that we need to be aware of?"

Witnesses reported that hostile fire brought down the latest helicopter, an AH-64 Apache attack helicopter that crashed Monday near Taji, killing its two pilots.

A journalist for Dubai-based Al-Arabiya television station, speaking from the scene, said the helicopter plunged to earth in a ball of flames after being hit by a rocket.

A little-known Islamist group, Salahu Din al-Ayubi (Saladin) Brigades, claimed in an Internet statement it used rockets to shoot down the Apache.

On Friday, a reconnaissance aircraft was brought down by what military officials said appeared to be hostile fire near Mosul in northern Iraq.

On January 8, a UH-60 Black Hawk helicopter carrying 12 people crashed in bad weather near the restive northwestern town of Tal Afar, killing all aboard. The dead included four civilians and eight military officials.

"Nothing has been ruled in our out in any of those three cases yet," DiRita said.

He said that despite witness accounts that the latest helicopter was shot down, reports were sketchy and investigators have not yet concluded what brought it down.

Brigadier General Edward Sinclair, the commander of the army's aviation center at Fort Rucker, Albama, disclosed that the latest threat against military helicopters in Iraq are "aerial IEDs."

He told Defense News that insurgents have been placing the improvised explosive devices along known flight paths and then triggering them when American helicopters fly in at rooftop level.

Sinclair said the bomb's builders may be using radio activated proximity fuzes taken from artillery, anti-aircraft and mortar shells to detonate the warhead at altitudes of about 15 meters (50 feet).

Defense News said the devices have been used against helicopters on multiple occasions, but the general would not say if they have caused damage.

Army helicopters come under insurgent fire 15 or 20 times a month, Sinclair said.

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