The four US helicopters that crashed in Iraq in the past two weeks were apparently shot down by insurgents, US military spokesman Major General William Caldwell said on Sunday.

This is the first time the US command has publicly acknowledged that the aircraft were lost to enemy fire, the media suggested Monday.

Caldwell said the probes into the crashes of three Army and one private helicopters are incomplete but "it does appear they were all the result of some kind of anti-Iraqi ground fire that did bring those helicopters down."

The incidents were not unusual as "there has been an ongoing effort since we have been here to target our helicopters," according to Caldwell.

He said 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."

On Friday, Genenal Peter Pace, chairman of the Joint Chiefs of Staff, acknowledged that insurgent ground fire in Iraq "has been more effective against our helicopters in the last couple of weeks."

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

Between Jan. 20 and Feb. 2, four US helicopters, three of them military aircraft, crashed in and around Baghdad, killing 20 people in all.

(Xinhua News Agency February 5, 2007)