BAGHDAD, Iraq — The American command has ordered changes in flight operations after four helicopters were shot down in the last two weeks, the chief military spokesman said yesterday, acknowledging for the first time that the aircraft were lost to hostile fire.

The crashes, which began January 20, follow insurgent claims that they have received new stocks of anti-aircraft weapons — and a recent boast by Sunni militants that “God has granted new ways” to threaten American aircraft.

All four helicopters were shot down during a recent increase in violence, which an Interior Ministry official said has claimed nearly 1,000 lives in the past week alone. At least 103 people were killed or found dead yesterday, most of them in Baghdad, police reported.

Major General William Caldwell told reporters that the investigations into the crashes of one private and three Army helicopters were incomplete but that “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 American Iraq command had said publicly that all four helicopters were shot down.