FALLUJAH, Iraq - U.S. warplanes and artillery bombarded southern parts of Fallujah where troops were trying to squeeze Sunni fighters in a smaller and smaller cordon today. The military estimated 600 insurgents have been killed in the offensive but acknowledged success in the city won't break Iraq's insurgency.

The huge Fallujah campaign has also sent a stream of American wounded to the military's main hospital in Europe. Planes carrying around 90 bloodied and broken troops were expected today at Landstuhl Regional Medical Center in Germany. They join 125 wounded soldiers flown there already this week.

At least 13 U.S. soldiers and Marines have been killed so far in the Fallujah operation, according to military reports pieced together since Monday.

Two Marine Super Cobra attack helicopters were hit by ground fire and forced to land in separate incidents near Fallujah, the military said today. The crews were not injured and were rescued.

U.S. troops were steadily advancing through the city from the northern side, pushing militants slowly into the southern half. With U.S. units positioned to the south and east, and the Euphrates River on the west, insurgents are being squeezed into a corner, the military said.

Loud explosions rocked the city throughout the morning as gunfire reverberated across town and helicopters hovered overhead.

American troops, on the verge of gaining control of the city, have been fighting pockets of resistance in this former militant stronghold as an Iraqi commander reported the discovery of "hostage slaughterhouses" in which foreign captives had been killed. Documents of hostages were found, along with CDs showing beheadings and the black clothes of kidnappers, he said.

U.S. troops discovered an Iraqi man chained to a wall in a building in northeastern Fallujah, the military said today. The man, who was shackled at the ankles and wrists, bruised and starving, told Marines he was a taxi driver abducted 10 days ago and that his captors had beat him with cables.

Three other Iraqi captives - contractors who had been working on U.S. bases and had been abducted a week ago - were also found, BBC embedded correspondent Jennifer Glasse said.

The three men were found blindfolded, handcuffed and in a locked room. Six suspected militants were detained in the raid on the building, where Marines also uncovered surface-to-air missiles, night-vision equipment, black uniforms, computers, weapons and videos showing torture of hostages.

In one of the most dramatic clashes Wednesday, snipers fired on U.S. and Iraqi troops from the minarets of the Khulafah al-Rashid mosque, the military said. U.S. Marines called in an airstrike, and an F-18 dropped a 500-pound bomb on the mosque, destroying both minarets.

Pool footage showed U.S. forces battling insurgents in a neighborhood surrounding the mosque. Troops were pinned down by gunfire on a rooftop, forced to hit the deck and lay on their stomachs.

"When they're using a mosque to do command and control for insurgents and kill my fellow Marines and soldiers and airmen that are out here - no holds barred, the gloves are off," said Marine Staff Sgt. Sam Mortimer.