A Nov. 13 article on the U.S. military offensive in Fallujah, Iraq, gave an incorrect number of wounded troops who subsequently returned to battle. The correct number was 40. (Published 11/15/04)

Insurgents in trenches met advancing U.S. and Iraqi forces in southern Fallujah with a burst of bullets and rockets Friday in what commanders described as one of the fiercest days of fighting since the battle to retake the city began five days ago.

Marines and soldiers said they encountered guerrillas dug into traditional defensive positions from which they could pop up, shoot and quickly take cover. The Americans said they and their Iraqi allies fought back with rifles, automatic weapons, belt-fed machine guns, mortars and hand grenades.

"It was a hornet's nest," said Capt. Erik Krivda, of Gaithersburg, the officer in charge of the Army's 1st Infantry Division Task Force 2-2 tactical operations command center.

Military officials also reported that fighting had resumed Thursday night in Fallujah's Jolan neighborhood, an insurgent stronghold in the city's northwest. Elsewhere in Iraq, intense fighting continued for a third day in the northern city of Mosul and other flash points in Iraq's Sunni Muslim heartland.

Lt. Gen. John Sattler, the Marine commander in Iraq, said 22 U.S. troops have been killed and more than 170 seriously wounded in and around Fallujah since the offensive began Monday night. An additional 490 troops suffered wounds but were able to return to duty, he said.

In addition, Sattler said, five members of the Iraqi security forces have been killed and 40 wounded.

At a Marine outpost near the city, a steady stream of ambulances carried casualties to a naval field hospital where troops lay on stretchers, their wounds covered by white gauze.

Since moving into Fallujah on Monday, U.S. forces have largely gained control of the city's northern half while driving insurgents south. The U.S. military said it now controls about 80 percent of the city.

Sattler said Friday that U.S. and Iraqi forces had broken the insurgents' "back and spirit. The goal right now is to continue to keep the heat on them. The concern now is to take care of this fight, reestablish the rule of law and return the town to the Fallujah people."

Commanders had warned, however, that insurgents might try to make a last stand in southwestern districts.

By midmorning, after reportedly taking heavy casualties, the units trying to capture the area called in artillery and air support, unleashing a barrage of shells and bombs that engulfed the southern neighborhood in flames and smoke. Witnesses reported another big battle in central Fallujah at the Rawtha Mohammediya mosque, which had served as the insurgents' headquarters but is now controlled by the Marines. About 200 to 300 fighters came from southern neighborhoods to stage the assault, but it ended after two hours with their suffering heavy losses, according to witnesses.

Insurgents had returned late Thursday to the Jolan neighborhood, where they engaged Marines for more than six hours, long enough to gather for Friday prayers in its Maathid mosque.

U.S. and Iraqi forces have detained 450 suspected insurgents. Thaer Hasen Naqib, spokesman for interim Prime Minister Ayad Allawi, said they included men from Iran, Saudi Arabia, Egypt, Jordan and Syria.

"It really doesn't matter from which group they are," Naqib said during a news conference at a military outpost near Fallujah. "They are foreigners. They are not invited to come to Iraq. We want to get rid of them as soon as possible."

In Mosul, Iraq's third-largest city, Iraqi authorities and U.S. forces were struggling to maintain control as insurgents moved at will through large sections of the city, residents said. The military said 10 Iraqi National Guardsmen and one American soldier were killed in Mosul on Thursday.

Brig. Gen. Carter Ham, who commands U.S. forces in Mosul, said combat was "sporadic" Friday and less intense than the day before. Still, the situation was deemed sufficiently difficult that an Army light-armored unit was peeled away from Fallujah to reinforce the U.S. force in Mosul.

The provincial governor called for massive reinforcements to supplement the Mosul police force, which splintered under a wave of insurgent attacks on at least five police stations Thursday. Iraqi National Guard units were being rushed to the city from three directions, as were Kurdish forces from Irbil to the south, the Associated Press reported. The offices of Kurdish political parties were among the buildings attacked in Mosul on Friday.

"We asked the central government in Baghdad, and God willing, they should arrive today," said the provincial governor, Duraid Kashmoula.

He said insurgents had penetrated the local security forces, hastening their partial collapse. Iraq's Interior Ministry fired the city's police chief, Brig. Gen. Mohammed Kheiri Barhawi.

Ham, the American commander, said in an interview with the BBC that "some police did not perform as well as we might like." He told CNN: "It's fair to say there are some with ties to the insurgents. We'd be kidding ourselves if we thought that was not the case."

Insurgents also launched attacks this week in smaller cities across Iraq's midsection, including Baqubah, a restive provincial capital northeast of Baghdad.

Bands of armed men continued to operate in Baghdad. Clashes were reported in the suburb of Abu Ghraib and the west Baghdad neighborhood of Ghazaliya. South of Baghdad, a U.S. soldier was killed and three people were wounded when insurgents attacked a patrol with a roadside bomb, rifle fire and rocket-propelled grenades.

An overflow crowd of worshipers cheered the American setbacks at Baghdad's largest Sunni mosque, where Friday prayers ended with grenade explosions, rifle fire and cries of "Allahu Akbar!" or "God is great!"

"Maybe you are not aware that Mosul is now under the control of the resistance, and all of the province of Anbar beyond Fallujah," said Mohammed Bashar Faydhi, who delivered the sermon. "The Americans have to realize that they need 25 million soldiers to defeat this population of 25 million Iraqis. They don't realize that the more oppressive they become, the more the resistance will grow."

Abu Musab Zarqawi, the Jordanian who officials say turned Fallujah into a nerve center for terrorist attacks across Iraq, issued a five-minute audiotape urging on insurgents in Fallujah. His organization, now known as al Qaeda in Iraq, accounts for a significant share of the fighters in the city, but officials said they believed Zarqawi left Fallujah before the fighting began.

"The banner of the jihad has been raised and is waving," he said on the tape. "The arms of the heroes of Islam have grown stronger in Iraq, and the hearts of the people of Islam are pounding with joy and awaiting a growing hope."

Vick reported from Baghdad. Special correspondent Omar Fekeiki contributed to this report.