Those four helicopters were all believed to have been shot down, the military has said, raising new questions about whether Iraqi extremists are using more sophisticated weapons or whether U.S. tactics need changing.
Any fresh threat to aviation would present serious problems for U.S. commanders as they launch the new security crackdown in Baghdad.
The U.S. military relies heavily on helicopters in Iraq, not only for supporting ground forces in combat but also to move troops and equipment by air to avoid roadside bombs and insurgent ambushes.
At night, U.S. attack helicopters prowl the darkened skies over Baghdad and other cities, using night-vision equipment to hunt for insurgents and militiamen planting roadside bombs or setting up firing positions.
Gen. Peter Pace, chairman of the Joint Chiefs of Staff, has acknowledged that insurgent ground fire in Iraq has been increasingly effective.
"I do not know whether or not it is the law of averages that caught up with us or if there's been a change in tactics, techniques and procedures on the part of the enemy," Pace told a Senate committee Tuesday.
In December, a spokesman for Saddam Hussein's ousted Baath party, Khudair al-Murshidi, told The Associated Press in Damascus, Syria, that Sunni insurgents had received new stocks of shoulder-fired anti-aircraft missiles and "we are going to surprise them."
However, Pace said Tuesday that the four helicopters that crashed since Jan. 20 were brought down by small arms fire rather than missiles.
On Wednesday, the Stars and Stripes newspaper quoted a U.S. officer as saying insurgents brought down an Apache helicopter last week near Taji by concentrating heavy automatic fire on the aircraft - rather than by using a shoulder-fired missile.
Helicopter transmission gears and rear rotor assemblies are vulnerable to ground fire and cannot be protected by armor plating because of the weight.
Sea Knight helicopters, with their distinctive dual rotors, are used by the Marines primarily as a cargo and troop transport, and can carry 25 combat-loaded troops, according to the think tank GlobalSecurity.org.
AP military writer Robert Burns contributed to this report from Washington.
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