Baghdad - The US military is adjusting its tactics in Iraq after four helicopters were shot down over the past two weeks, US military spokesman Major General William Caldwell said on Sunday.

Caldwell said the helicopters had been shot down in four separate incidents in which 21 US servicemen and private security contractors were killed, confirming earlier witness reports and leaks from within the US military.

Dozens of US helicopters have come down, some hit by missiles or gunfire, in four years of fighting. But the unusually high number lost in such a short time raised questions about whether militants had changed tactics or were using more sophisticated weapons.

"There has been an ongoing effort to target our helicopters," 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 said the incidents were still under investigation, but in the meantime US helicopters were changing the way they flew 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," Caldwell said.

An al Qaeda-backed group -- the Islamic State in Iraq -- on Sunday issued a video which it said showed its fighters downing a US helicopter northwest of Baghdad. The crash on Friday, the latest such incident, killed the helicopter's two crew.

The video, posted on a Web site used by insurgents, showed masked militants firing on a low-flying helicopter before firing what appeared to be a missile at it.

The craft, trailing smoke, flew away as militants chanted "Allahu Akbar" (God is Greatest), then disappeared behind a hill. The video, which could not be authenticated, later showed billowing smoke apparently caused by the crash.

The US military relies heavily on helicopters to transport troops or attack suspected militants holed up in buildings.

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, and a fifth contractor was shot on the second helicopter, the US embassy said in a statement at the time.

An Apache was hit by machinegun 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.

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