BAGHDAD, Iraq - U.S. jets and helicopter gunships launched the biggest air operation in central Iraq since major combat ended, blasting suspected ambush sites and hideouts yesterday with 500-pound bombs. Explosions rocked western Baghdad as American troops mounted fresh attacks against insurgents.

While the U.S. military stepped up its campaign to stamp out Iraqi insurgents, it also claimed progress on another front - preventing foreign fighters from entering Iraq from neighboring countries to carry out attacks on American forces.

Maj. Gen. Charles H. Swannack Jr., commander of the 82nd Airborne Division, said the number of U.S. soldiers in Anbar province, bordering Syria, Jordan and Saudi Arabia, has been tripled in the past two months, to 20,000. That, he said, has curbed infiltration.

"We are not fighting foreign fighters coming across the border in significant numbers," Swannack said. "We are fighting mostly ... locals" loyal to Saddam Hussein's ousted regime.

Insurgents struck again yesterday, wounding two U.S. soldiers with a roadside bomb in the northern city of Mosul, the military said. The military said a U.S. civilian contractor was killed Monday by a land mine near Baghdad.

The air activity yesterday was centered around Baqouba, 30 miles northeast of Baghdad. U.S. jets and Apache helicopter gunships blasted abandoned buildings, walls and trees along a road where attacks have been so common that troops nicknamed it "RPG Alley" after the rocket-propelled grenades used by insurgents.

Fighter-bombers dropped 500-pound bombs and tanks fired their 120 mm guns at suspected ambush sites, the military said.

Elsewhere, F-16 fighter aircraft bombed insurgent targets near the town of Samara, about 60 miles north of Baghdad, the military said.

U.S. troops fired mortars late yesterday on areas used by insurgents to launch mortar and rocket attacks against coalition forces in another night of huge explosions in Hussein's hometown of Tikrit.

A group of Bradley fighting vehicles and armored personnel carriers fired at a bunker that was part of Hussein's former military defenses south of the town and an outlying farm to the north.

Lt. Colin Crow, who oversaw the strike, said the targets were uninhabited and that the attacks were meant to keep insurgents from using them as platforms for assaults.

"Basically, we're kind of claiming the ground that the enemy is using at us," he said. "They have to move further and further out."

Meanwhile, U.S. officials provided more details of an attack carried out Sunday. An Army short-range missile hit a house south of Tikrit owned by Izzat Ibrahim al-Douri, a former Iraqi official the U.S. military accuses of being behind many attacks on coalition forces, a Defense Department official said, speaking on condition of anonymity. There was no indication that anyone was inside at the time.

The stepped-up military operations followed an escalation in insurgent attacks over the past three weeks.

In response, the U.S. military announced Operation Iron Hammer, aimed at striking at suspected rebel targets before insurgents have the chance to attack.

The strategy appeared aimed at showing U.S. resolve as Washington prepares to hand over political power to a new Iraqi provisional government by the end of June. However, the use of heavy force could further alienate an Iraqi population already chafing under foreign military occupation.

During a news conference in Baghdad, Swannack, whose division is responsible for Anbar province, said the robust tactic "demonstrates our resolve."

"We will use force, overwhelming combat power, when it's necessary," he said.

Swannack, whose troops patrol such hot spots as Fallujah, Ramadi and the borders with Syria and Saudi Arabia, said he believes that most of the insurgents are Iraqis.

"Ninety percent of the cases are from regime loyalists and [Iraqi] Wahhabis," he said.

Wahhabis are members of a strict Islamic sect that dominates Saudi Arabia and has followers in Iraq.

He said seven foreign fighters were killed recently in Anbar province and 13 were captured. He did not have their nationalities or other details.

At the same time, U.S. forces are pulling out of some hot spot towns to hand over security powers to Iraqis.

Swannack said he will withdraw his troops from the center of Ramadi, capital of Anbar province, by the first of the year, after a pullout from the town of Samara last weekend.

Swannack said U.S. forces would be able to move in quickly if problems arose. But the Iraqi civil defense chief in Samara pleaded for the Americans' return.

"We cannot handle this on our own," said Capt. Ihsan Aziz, adding that looters and pro-Hussein guerrillas could move in to fill the vacuum.

Paratroopers searched neighborhoods in Ramadi late Monday, detained about a dozen people and seized explosives and other materials for making roadside bombs, a major threat to American troops, the 82nd Airborne Division said in a statement.

One man was arrested after U.S. troops found "jihad signup sheets" in his house. The man, who was not identified, was suspected of financing and supervising the placement of roadside bombs around the city, the division statement said.

In Mosul, the 101st Airborne Division seized two weapons caches late Monday during an operation around the Qayyarah West Airfield, the military said.