A massive U.S. campaign to crush resistance by supporters of the ousted Saddam Hussein (searchregime entered its fourth day Friday with U.S. officials saying a large number of Iraqi fighters had been killed.

Air Force Gen. Richard B. Myers (searchsaid U.S. forces were sifting through intelligence that "foreign fighters" may have been at an alleged terrorist training camp northwest of Baghdad (searchtargeted by U.S. forces.

If true, it would be the first indication that volunteers from other Arab countries were still in Iraq since the war was declared effectively over on May 1. Before the war in March, Iraq claimed that thousands of Arab fighters poured into the country to resist the invasion. They provided some of the stiffest resistance once American forces entered Baghdad.

Six U.S. soldiers have been wounded in the past 24 hours in fighting in all of Iraq, said Capt. John Morgan, spokesman in Baghdad for the Army's V Corps (search).

U.S. military officials in Baghdad declined to give details of Friday's activities, saying the operations were ongoing and more fighting was possible. The operations are taking place in the heartland of Saddam Hussein's supporters -- where militants have stepped up ambushes and sniping at coalition forces.

"It's definitely an upsurge in large-scale operations. For as long as they are shooting at us, we'll continue these operations," said Sgt. 1st Class Brian Thomas, a U.S. military spokesman in Baghdad.

On Thursday, American warplanes bombed an alleged training facility 90 miles northwest of Baghdad, looking for members of the now-banned Baath Party, Iraqi paramilitary groups and "other subversive elements," said a military statement.

A fierce ground battle followed the air strike in which the Iraqi forces suffered heavy casualties. One American soldier was wounded, said the U.S. Central Command.

"It was a tough fight. They were well-trained or well-equipped, and clearly well prepared for this, for the fight they had," Myers said at a Washington briefing on Thursday.

"There were many killed -- a large number," Myers said.

"This is one of the many types of groups that we're going to have to confront, I think, for some time to come."

Also on Thursday, Iraqi fighters shot down an Apache 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 crashed Thursday. The crews of the aircraft were rescued unharmed.

Earlier this week, U.S. forces launched a sweep through towns of the so-called "Sunni triangle" north of and west of Baghdad in central Iraq and marked at its top by Tikrit, Saddam's hometown.

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, said Sgt. Forest Geary of the U.S. Army's 3rd Infantry Division.

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.

"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."

Military spokesmen said the campaign around Duluiyah -- dubbed Operation Peninsula Strike -- had ended by Friday.

As part of the effort to root out militants, 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.

U.S. military officials also said two Iraqi prisoners were shot trying to escape from a camp Thursday. One of the men later died of his wounds and the other was recaptured, U.S. Central Command said.

The United States is holding more than 2,000 Iraqis, including more than half of the 55 Iraqi most-wanted by Washington.