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

While the military stepped up its campaign to put down anti-U.S. guerrillas, it also claimed progress on another front — preventing foreign fighters from entering Iraq from neighboring nations 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 infiltrations.

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

The air activity Tuesday 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 battle tanks fired their 120mm 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 Tuesday on areas used by insurgents to launch mortar and rocket attacks against coalition forces in another night of huge explosions in Saddam’s hometown of Tikrit. One group of Bradley fighting vehicles and armored personnel carrier fired 17 mortar rounds toward a bunker that was part of Saddam’s former military defenses south of the town and an outlying farm to the north.

Lt. Colin Crow, who oversaw the mortar firing, said the targets were uninhabited and the attacks were meant to scare 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.”

The stepped-up operations follow 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.