The main thrust was into an area just north of Baghdad where intelligence indicated a remnant pro-Saddam power base.
It was a massive security sweep, sealing off towns, searching for weapons, blasting into houses. Almost 400 people were arrested, reports CBS News Correspondent Mark Phillips.
Although there have been no reported American casualties in the operation so far, one Apache helicopter was lost when it was shot down during combined land and air assault on what the military is calling a "terrorist camp" full of foreign fighters. The two uninjured, downed pilots were rescued.
An F16 fighter also crashed due to mechanical failure, the military said. It's pilot was rescued as well.
The raid came as troops for the third day engaged in "Operation Peninsula Strike," one of the largest military actions since the end of the war.
In other developments:
Ten to 15 Iraqis were killed in Thursday's action, part of a sweep through the so-called ``Sunni triangle'' north and west of Baghdad in central Iraq and marked at its top by Tikrit, Saddam's hometown.
"`As we receive actionable intelligence, we strike hard and with lethal force,'' Lt. Gen. David McKiernan, the U.S. ground forces commander in Iraq, said in a briefing Thursday. ``Iraq will be a combat zone for some time.''
Militants in recent weeks have stepped up ambushes and sniping at coalition forces in the triangle, a heartland of support for Saddam's now-banned Baath Party.
The southwestern corner of the "Sunni triangle" is the city of Fallujah, where four American soldiers have been killed and 21 wounded by Iraqi insurgents since April. As part of the ongoing crackdown, some 1,500 troops from the Army's 3rd Infantry Division were sent to Fallujah, which has about 200,000 residents, and the neighboring towns of Khaldiyah and Habaniyah.
The downed AH-64 Apache helicopter — was the first aircraft shot down by ground fire since Saddam was ousted two months ago — belonged to the U.S. Army's 101st Airborne Division, McKiernan said. A pair of Apaches fired on "irregular forces" at the crash scene, while U.S. ground troops secured the site and rescued the two-man crew.
In a related effort, Bremer banned gatherings, pronouncements or publications that incite disorder or violence against the U.S.-led occupation forces, or the return of the Baath Party.
Of the hundreds of Iraqis captured, Fitzgerald said he had no information whether any were among the top 55 most-wanted members of Saddam's former regime. He said U.S. officials armed with intelligence on particular suspects were questioning those captured.