He said the US was "adjusting" its use of helicopters while the incidents were investigated.
 

"Based on what we've seen, we are already adjusting our tactics and procedures in how we deploy our helicopters," Caldwell said.

 

The US military relies heavily on helicopters to transport its troops or launch air strikes against suspected fighters holed up in buildings.

 

Changing tactics

 

Dozens of US helicopters have come down, some of them hit by missiles or gunfire, in four years of fighting.

 

But the unusually high number of helicopters lost in such a short time had raised questions about whether fighters had changed tactics or were using more sophisticated weapons.

 

A Black Hawk helicopter carrying 12 US soldiers and crew crashed northeast of Baghdad on January 20, amid suggestions that it had been brought down by a shoulder-fired missile.

 

Three days later, two helicopters flown by Blackwater security contractors were attacked while coming to the aid of US embassy personnel in central Baghdad.

 

One helicopter crashed under heavy gunfire, killing all four on board, while a fifth contractor was shot on the second helicopter, the US embassy said in a statement at the time.

 

An Apache was hit by machine-gun fire and its two crew killed while supporting Iraqi troops battling heavily armed followers of a messianic Muslim cult near Najaf last weekend.

 

In the latest incident, a helicopter crashed northwest of Baghdad on Friday, killing its two crew.

 

Website video

 

Al Jazeera aired a video late on Sunday showing one of the US helicopters being hit in central Iraq and said it came from an anti-government group's website.

 

The channel showed a grainy 15-second video, filmed from the ground, which showed a helicopter plunging with a trail of black smoke emanating from it.

 

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Al Jazeera said the video was of the Apache hit on Friday near Taji in central Iraq.

 

The authenticity of the video could not be immediately verified.

 

In December, a spokesman for Khudair al-Murshidi, Saddam Hussein's ousted Baath party, told The Associated Press in Damascus, Syria, that Sunni fighters had received shoulder-fired anti-aircraft missiles and "we are going to surprise them", meaning US forces.

 

Al-Murshidi did not say when or how the missiles were obtained.

 

Fighters have used SA-7s, a shoulder-fired missile with an infrared homing device, against US and British aircraft since 2003.