The United States military has confirmed for the first time that four helicopters that crashed in Iraq in the past two weeks were all shot down.
Major-General William Caldwell says US forces are adjusting their tactics and changing the way in which attack helicopters fly in support of Iraqi and American troops.
He says the helicopters were shot down by ground fire in four separate incidents in which 21 US servicemen and private security contractors were killed.
In four years of fighting, dozens of US helicopters have come down, some of them hit by missiles or gunfire.
But the unusually high number of helicopters lost in such a short time has raised questions about whether militants have changed tactics or are using more sophisticated weapons.
"There has been an ongoing effort to target our helicopters," Maj-Gen Caldwell told reporters in Baghdad.
"We have had four helicopters shot down ... It appears they were all the result of some kind of ground fire."
He says the incidents are still under investigation, but in the meantime US helicopters are changing they way they fly in support of Iraqi and American troops.
"Based on what we've seen, we are already adjusting our tactics and procedures in how we deploy our helicopters," he said.
The US military relies heavily on helicopters to transport its troops or launch air strikes against suspected militants holed up in buildings.
A Black Hawk helicopter carrying 12 US soldiers and crew crashed north-east 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 the holy city of Najaf last weekend, in an incident witnessed by a Reuters reporter.
In the latest incident, a helicopter crashed north-west of Baghdad on Friday, killing its two crew.
