Balad, Iraq ? U.S. troops in Apache helicopters and Bradley armored vehicles fought attackers Friday after Iraqis hiding in a thicket of reeds rushed a U.S. tank patrol.

The battles marked day four of a U.S. sweep to hunt down supporters of Saddam Hussein’s fallen regime. About 100 anti-American fighters have been killed in the sweep, the biggest U.S. military operation since Saddam’s regime fell.

Staged by the 3rd and 4th Infantry Divisions, the fighting follows the U.S. strategy of luring opposition holdouts into the open, where their assault rifles and rocket launchers are no match for American forces equipped with computerized weaponry and heavy armor.

The Iraqi ambush started just before midnight Thursday, when a large force of insurgents detonated a land mine and fired rockets on a patrol of the 4th Infantry Division about 30 miles northwest of Baghdad, Lt. Col. Andy Fowler said.

The tanks returned fire, killing four assailants. The patrol called in reinforcements, including Apache helicopters, and gave chase to the fleeing attackers, killing 23, U.S. Central Command said. There were no American casualties.

Fowler, looking haggard, said the pursuit lasted through the night and into daylight Friday. Some attackers fled through sunflower fields and ducked into sand-brick houses.

Fowler, of the 3rd Infantry Division, said his patrols had been ambushed several times before, “but nothing like last night.” He added: “It was a very long night.”

U.S. strategists said the military had been preparing to strike the remnants of Saddam’s fighting forces since the end of the war, gathering intelligence on their whereabouts and capabilities and taunting them into action.

“We will maintain that pressure, causing him to react to us, rather than vice versa,” said Lt. Gen. David McKiernan, U.S. ground forces commander in Iraq. “Are there bad guys still out there? Absolutely. Are we going after them? Absolutely.”

Most of the opposition casualties happened Thursday.