Major General William Caldwell said the investigations into the crashes of three Army and one private helicopters were incomplete, but "it does appear they were all the result of some kind of anti-Iraqi ground fire that did bring those helicopters down."
It was the first time a senior figure in the US Iraq command had said publicly that all four helicopters were shot down.
Despite the losses, Caldwell said it was premature to conclude that the threat to US aircraft posed by Sunni insurgents and Shi'ite militiamen had increased dramatically.
"There's been an ongoing effort since we've been here to target our helicopters," Caldwell said. "Based on what we have seen, we're already making adjustments in our tactics and techniques and procedures as to how we employ our helicopters."
Caldwell did not elaborate, presumably for security reasons. In the past, defensive measures have included flying lower and faster, varying routes and using zigzag patterns over dangerous areas.
Three copters crashed in mostly Sunni areas and the fourth was shot down during fighting with Shi'ite cultists near Najaf. US officials have accused Iran of providing sophisticated weapons to Shi'ite militants.
In December, a spokesman for Saddam Hussein's ousted Ba'ath Party, Khudair al-Murshidi, said in Damascus that Sunni insurgents had received shoulder-fired antiaircraft missiles, and "we are going to surprise them," meaning US forces.
Murshidi did not say when or how the missiles were obtained.
Insurgents have used SA-7s, a shoulder-fired missile with an infrared homing device, against US and British aircraft since 2003.
In an Internet statement, the Al Qaeda-affiliated Islamic State of Iraq claimed responsibility for the latest crash -- an Apache Longbow helicopter that went down Friday north of Baghdad, killing two crew members.
"We tell the enemies of God that the airspace of the Islamic State in Iraq is prohibited to your aircraft just like its lands are," the statement said. "God has granted new ways for the soldiers of the State of Iraq to confront your aircraft."