U.S. forces pushed toward a corner of Fallujah where commanders said insurgents may be preparing to make a last stand, as soldiers and civilians uncovered evidence of atrocities committed by the foreign and Iraqi guerrillas who controlled the city for nearly seven months.

In the industrial area on Fallujah's south side, residents said Thursday that the bodies of 20 foreign fighters had been found outside a truck repair shop, many killed by a single shot to the head. Insurgents native to Fallujah said the foreigners were executed for deserting their positions when the U.S.-led assault on the city began Monday night.

In the northern half of the city, now largely under the control of U.S. and Iraqi forces, Marines making a door-to-door sweep on Wednesday found a bruised, starving man chained to the wall of a house. The man, who identified himself as a taxi driver from nearby Abu Ghraib, said he had been kidnapped by men who refused to give him food or water and beat him with electrical cords during 10 days of captivity.

Military commanders said Marine and Army units were continuing to battle pockets of insurgents throughout the city as they pushed toward Fallujah's southern residential districts. Troops on foot patrol traded fire with guerrillas, then scurried for cover behind concrete walls and buildings, returning fire that rang through the otherwise deserted streets.

The U.S. military said 18 of its troops had been killed and 178 wounded during four days of fighting in Fallujah. Five Iraqi troops were reported killed and 24 wounded in the same period.

A U.S. military spokeswoman said 102 seriously wounded soldiers from around Iraq had been flown to the main U.S. military hospital in Germany on Thursday, joining 125 who arrived Monday through Wednesday.

Numbers of insurgent and civilian casualties could not be independently determined, but a military spokesman in Baghdad, Lt. Col. Steve Boylan, said an estimated 600 rebels had been killed so far, the Associated Press reported.

The Reuters news agency reported that Defense Secretary Donald H. Rumsfeld, in El Salvador at the start of a week-long trip through Latin America, said that although some insurgents likely fled Fallujah before the offensive, "we also know that there are a number of hundreds that didn't, and have been killed. Others have been captured."

But officials cautioned that despite the large number of casualties, the insurgency would continue elsewhere. Bryan Whitman, a Defense Department spokesman in Washington, called the military's success in Fallujah "an important milestone" but said it by no means marked the end of the insurgency.

U.S. troops reported that Fallujah was laced with booby traps, including the rudimentary bombs the military calls improvised explosive devices, or IEDs, tucked into rubble and garbage. The troops reported uncovering large stockpiles of weapons, some of them hidden in mosques.

Maj. Gen. Richard F. Natonski, commander of the 1st Marine Division, said U.S. forces "respect the law of the war, unlike the other side, who uses mosques. In almost every single mosque in Fallujah, we've found an arms cache. We've found IED factories. . . . We've also seen the use of schools for the storage of weapons. This is the enemy that we fight. It doesn't respect the religious mosques or the children's schools."

Asked at a news conference at a camp outside Fallujah if troops expected to find more insurgents in the city, Natonski said yes, "And we will kill them."

Before the offensive began, Fallujah police announced that 157 civilian families remained in the city, whose population is normally about 250,000. On Wednesday, those who had survived the fighting found leaflets dropped by U.S. aircraft offering safe passage out of the city.

They emerged to the stench of burning flesh, on streets littered with broken bricks and scores of bodies, some subjected to such heat that they had melted. Dead fish floated on the Euphrates River, brought to the surface by mortar shells that insurgents had fired at U.S. positions on the river's western bank.

In a charity hospital operating in tents because its building was damaged by bombing, a young Arab fighter writhed in agony while blood seeped from his ears, eyes, nose and mouth. A doctor said the hospital, donated by the United Arab Emirates, counted 32 civilian wounded by Wednesday, including nine women and four children.

As civilians filed out of the city, scores of fighters put down their guns and joined them, residents said. Several told a witness that they were not quitting the war, but rather moving to open a second front in Baqubah, an insurgent hotbed northeast of Baghdad.

As the new refugees recounted the events of recent days and weeks, a picture of the battle from the insurgents' side began to emerge. Witnesses described an insurgency fractured by distrust and rivalries between locals and foreigners, and visibly shaken by the thunderous U.S. assault.

The foreigners found slain Thursday in southern Fallujah were described as foot soldiers with Monotheism and Jihad, a guerrilla group headed by Jordanian Abu Musab Zarqawi that now calls itself al Qaeda in Iraq. In the plans developed by insurgent leaders for a coordinated defense of the city, Zarqawi's fighters were to man bunkers in two neighborhoods, according to witnesses. Others were to be defended by various Iraqi insurgent groups, including the First Army of Mohammad and Ansar al-Sunna Army.

But residents said strains between the local insurgents and the foreigners quickly turned into a deep schism under the intense pressure of the U.S.-led offensive. When a senior Zarqawi commander was found dead of a bullet to the head during the battle, debate ensued over whether he was killed from a distance by a U.S. sniper or at close range by an Iraqi insurgent, residents said.

Residents said everyone in the city, including the insurgents, was stunned by the firepower the Americans brought to the battle. Guerrillas counted 40 armored vehicles approaching their positions as night fell Monday.

The insurgents suffered their worst single loss -- at least 50 dead -- counterattacking U.S. forces who had taken the Rawdha Muhammediya mosque that had served as the insurgency's headquarters, witnesses said. The witnesses said they also counted as many as 10 American bodies.

"The confrontation with the American Army, which is the most powerful military organization in the world, is itself a great victory for us," said Abdullah Janabi, head of the mujaheddin shura, the council that had ruled Fallujah as a self-appointed government since April. "We were proud enough that Fallujah . . . was able to fight and confront America for seven months and still force the Americans and the Iraqi government to sit down and negotiate."

The kidnap victim discovered Wednesday by Marines was shackled to a wall by his wrists and ankles, according to Maj. Francis Piccoli, a spokesman for the 1st Marine Expeditionary Force. When the Marines entered the house, the driver, who speaks little English, called out "Uncle, uncle" to communicate with the troops.

Video footage shot by an ABC crew showed the man, shirtless and wrapped in a wool Marine-issue blanket, saying through an interpreter that when his captors fled, he told them he would die without food or water. They responded: "We brought you here to die."

In the video footage, the man, still wearing handcuffs, said he had been kidnapped while walking through Abu Ghraib. Two men grabbed him and shoved him into a car, he said.

Vick reported from Baghdad. Staff writer Josh White in Washington contributed to this report.

Marines arrest men in the center of Fallujah, where U.S. forces battled pockets of insurgents.