A U.S. military helicopter crashed north of the Shiite holy city of Najaf in central Iraq Jan. 28, 2007. (Xinhua/AFP Photo) Photo Gallery>>>

    BEIJING, Feb. 5 (Xinhuanet) -- The four U.S. helicopters that crashed in Iraq in the past two weeks were apparently shot down by insurgents, U.S. military spokesman Major General William Caldwell said on Sunday.

    This is the first time the U.S. 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 U.S. 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 U.S. military relies heavily on helicopters to transport troops or attack suspected militants holed up in buildings.

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

    The Jan. 20 crash, in which 12 U.S. troops were killed when a military helicopter went down northeast of Baghdad, was one of the worst since the March 2003 invasion of Iraq by U.S.-led coalition forces.

    On Jan. 22, a U.S. Blackwater helicopter went down in Baghdad killing four U.S. private security guards.

    On Jan. 28, another military helicopter crashed north of the Shiite holy city of Najaf in central Iraq, killing two soldiers, while on Feb. 2 a military chopper crashed north of Baghdad, killing two more troops.

    The Iraqi Interior Ministry estimated about 1,000 people were killed throughout Iraq in the past week due to surging violent attacks.

    The statistics came just one day after one of the deadliest single bomb attacks in central Baghdad that killed at least 130 people and wounded some 305.

    (Agencies)