The incidents came as U.S. ground troops wound up a massive sweep in a Sunni Muslim enclave north of Baghdad, aimed at routing out the organizers of attacks on occupation forces. Thursday's events marked a sharp escalation of U.S. military operations in central and western Iraq, where guerrillas have intensified attacks on U.S. troops in recent weeks.

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

The downed AH-64 Apache helicopter belonged to the U.S. Army's 101st Airborne Division, a Central Command statement said. A pair of Apaches fired on "irregular forces" at the crash scene, while U.S. ground troops secured the site and rescued the uninjured two-man crew.

It was the first aircraft shot down by ground fire since Saddam Hussein was ousted two months ago. Central Command did not say exactly where it went down.

Hours earlier, at about 1:45 a.m., U.S. planes attacked a site they described as a terror camp 95 miles north of Baghdad, according to Central Command. A firefight broke out, and one coalition soldier was slightly injured. The statement did not give any Iraqi casualty figures.

Meanwhile, in the third day of an assault dubbed "Operation Peninsula Strike," thousands of American troops swept through an area centered on the Tigris River town of Duluiyah, 45 miles north of Baghdad.

Ten to 15 Iraqis were killed in the sweep, and four U.S. soldiers suffered gunshot wounds, said U.S. Sgt. Forest Geary. Three of the injured Americans were flown to Germany for medical care, he said.

Fighter jets, attack helicopters and unmanned aerial drones backed up ground troops during the operation, in which about 400 people have been captured in three days of strikes.

Fitzgerald said he had no information on the capture of any of the top 55 most wanted members of Saddam's regime. He said U.S. officials armed with intelligence on particular suspects were still questioning those captured. Prisoners deemed not hostile will be released, he said.

Interrogators are "working with information that has directed the finger toward these suspects," Fitzgerald said. "If we believe they're dangerous and will cause problems for the Iraqi people or coalition forces, we'll keep them for further information."

The heavily wooded area provided good cover for ambushers, but the searches failed to turn up more than a few light arms and rocket propelled grenades, weaponry commonly found all over Iraq, said troops involved in the operation.

"When we came down here we really expected the worse. That has not been the reality," said Sgt. Todd Oliver, of the 173rd Airborne Division. "We came here searching houses and knocking down doors. If they were here, they're gone now."

No Americans have been killed in the operation, Fitzgerald said.

The region north and west of Baghdad is part of the so-called Sunni triangle, the heartland of support for Saddam's now-banned Baath Party and not far from ex-leader's hometown of Tikrit. Saddam is a Sunni Muslim.

Some 1,500 troops from the U.S. Army's 3rd Infantry Division were sent to Fallujah, a Sunni city of some 200,000 people, and the neighboring towns of Khaldiyah and Habaniyah.

Since U.S. forces entered Fallujah in April, four American soldiers have been killed and 21 wounded by insurgents and U.S. troops have killed at least 23 Iraqis and wounded 78 wounded.

In Habaniyah, a top U.S. commander said his men have made significant progress in restoring security.

"There are three elements we are having to deal with, first armed bandits, second former Baath Party officials are paying people to attack us, and then the Fedayeen," said Maj. Gen. Buford Blount III, commander of the 3rd Infantry Division. The Fedayeen were a paramilitary force set up by Saddam's regime.

U.S. intelligence has made progress in figuring out which groups are responsible for which attacks and U.S. troops are working to dismantle them, Blount said.

Anecdotal evidence such as large amounts of cash seized during arrests of militants suggests that someone is paying the militants to attack U.S. troops, said Sgt. Brian Thomas, a U.S. Army spokesman in Baghdad.

The attackers have used guerrilla tactics allowing them to strike U.S. military vehicles and escape. They also appeared to be coordinating raids with signaling devices, including flares, military officials said.

Duluiyah, largely untouched during the war, is said to be a likely place of refuge for Saddam die-hard fighters.

In another development Thursday, Central Command said that a U.S. F-16 fighter-bomber crashed early southwest of Baghdad.

The statement said the pilot ejected safely and was rescued by ground forces. It said the cause of the incident was being investigated.