Updated

U.S. fighter jets bombed a suspected terrorist camp and troops stormed door-to-door through Sunni Muslim (search) towns Thursday, seeking Saddam Hussein loyalists in one of the biggest American military assaults since the war.

As Operation Peninsula Strike (search) entered day three, Iraqi fighters shot down a U.S. helicopter gunship -- the first American aircraft downed by ground fire since Saddam's ouster two months ago -- and a U.S. F-16 fighter-bomber (search) crashed. The crews of the aircraft were unharmed.

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 (search).

Coalition forces did not give a total of Iraqi casualties in the operation, but said about 400 Iraqis have been arrested and many were being interrogated. No Americans have been killed, but four U.S. soldiers suffered gunshot wounds Thursday, said Sgt. Forest Geary, of the 37th Calvary, a unit of the 3rd Infantry Division (search).

"It's one of the largest operations since the war," U.S. Central Command (search) spokesman Lt. Ryan Fitzgerald said of the sweep.

As part of the effort, the American civilian administrator of Iraq, L. Paul Bremer, on Thursday banned gatherings, pronouncements or publications that incite disorder or violence against the U.S.-led occupation forces, or the return of the Baath Party.

After the downing of the attack helicopter, two other AH-64 Apaches provided cover fire against "irregular forces" while ground troops moved in to rescue the two-man crew, Central Command said.

The military would not specify where or when the attack happened, but McKiernan said the aircraft were engaged in an ongoing operation in which U.S. forces "struck very lethally and very decisively." The crash of the F-16 was under investigation.

Earlier Thursday, at about 1:45 a.m., U.S. warplanes attacked what the Central Command called a "terrorist training camp" 95 miles north of Baghdad, and followed up with a ground assault later in the morning.

Hundreds of U.S. troops moved in hard and fast through the area, centered on the town of Duluiyah 30 miles north of Baghdad. With helicopters whirring overhead and tanks offering cover, they kicked down doors and pulled out residents, looking for snipers who had harassed them for weeks from the shelter of thick woods.

Troops rounded up hundreds of people for questioning, though most young people were freed within hours. The U.S. military did not give details about the camp or why it was designated as "terrorist."

"During the day, the people are calm and friendly, but at night they've been ambushing us," said Geary.

The aggressive raids angered people in Duluiyah, who complained of needlessly heavy-handed tactics by the Americans. One man said his 6-year-old son was handcuffed.

In a mourner's tent on a side street of the mostly shuttered town, Abid Ali Jassem al-Juburi, a former general in Saddam's army, said he was grieving for his brother and cousin who died early in the U.S. operation.

"My brother was beaten, hit in the face and was killed," he said, adding that U.S. troops took away medicine his family was bringing for a cousin who had suffered a heart attack "and smashed it under their feet."

Another resident, Ammar Salim, said 31 members of his family, males aged 13 to 70, were detained. "There was no reason for them to be arrested. They did nothing," he said. "They (U.S. forces) destroyed all our furniture, all our belongings."

Specialist Chris Rossi, of the 173rd Airborne Brigade, manning a checkpoint on the Tigris River, was unapologetic.

"We're just not taking any chances," he said. "My life's in danger so I'm going to approach them (the Iraqis) as hostile. And until I'm proved otherwise, that's the way I'm going to approach it."

A U.S. military spokesman contacted in Baghdad said no senior officer was immediately available to comment on the townspeople's accusations.

McKiernan, the U.S. commander, said the search for insurgents was based on intelligence tips. Although action in the area appeared to be easing off, McKiernan told journalists in Baghdad that Operation Peninsula Strike would continue "as long as we have actionable intelligence."