NEAR FALLUJA, Iraq, Nov. 18 - American military officials said Thursday that they had discovered a house in the devastated city of Falluja that appeared to have been a headquarters for guerrillas of the Jordanian militant Abu Musab al-Zarqawi.
An American commander also said the weeklong offensive to take the city had "broken the back of the insurgency."
Despite that assessment, gun battles and mortar fire continued to shake the city, and the commander, Lt. Gen. John Sattler of the First Marine Expeditionary Force, said it would be "some time" before it was safe enough to allow many of Falluja's 300,000 residents to return. A wave of assaults continued across areas of central and northern Iraq dominated by Sunni Arabs, who controlled the country under Saddam Hussein.
The Shiite regions have remained relatively quiet. During the aborted American assault on Falluja last April, fighters loyal to the Shiite cleric Moktada al-Sadr launched attacks in southern Iraq and in Baghdad. The Shiites are looking to seize majority rule through elections in January, while hard-line Sunnis keep up the guerrilla war to prevent that.
The building that seemed to be a Zarqawi command center was taken by Americans on Sunday after a fierce gun battle with up to 25 well-trained insurgents, said Maj. David Johnson of the First Infantry Division, who accompanied the Army unit that searched the house on Thursday after being alerted by Iraqi soldiers.
The Iraqi soldiers had discovered letters they believe were exchanged between Mr. Zarqawi and his lieutenants, the major said, along with computers, materials for car bombs, identification papers (including a Sudanese passport) and medical supplies from the International Red Cross and the United States Agency for International Development. A classroom had a blackboard with drawings of American fighter jets.
Mr. Zarqawi's network is believed to be responsible for ambushes, bombings and beheadings that have left hundreds dead across Iraq since the summer of 2003, and the recent assault on Falluja was partly intended to destroy the most important base for the insurgency. American commanders say Mr. Zarqawi, who has a $25 million bounty on his head, almost certainly fled the city in the days or weeks leading up the offensive.
Marines continued to engage in firefights on Thursday with pockets of insurgents in southern Falluja. A marine and an Iraqi soldier were killed at sunset when they came under fire as they were trying to clear a building.
So far, at least 51 American servicemen have been killed and 425 wounded in the city since the began on Nov. 8, General Sattler said at a news conference at the Marine headquarters. Eight Iraqi soldiers have been killed and 43 wounded. About 1,200 insurgents appear to have been killed, he said.
The offensive had crippled the insurgents and "disrupted them around the country," he said.
But violence continued across central and northern Iraq. Bombings in Baghdad and two northern cities killed at least eight Iraqis. In Mosul, pushed to the brink of chaos by a revolt last week, rebels attacked a police station and lobbed 10 mortar shells at the provincial government center, wounding at least four of the governor's bodyguards. The Iraqi government was investigating reports that 63 freshly trained police had been abducted at gunpoint as they were driving in from Jordan.
In Basra, in the south, about 300 Shiite Arabs have banded together into a group called the Anger Brigades to battle extremist Sunni Arabs, Ali al-Mahdi, a spokesman for the group, said in an interview. The founding of the group raised the specter of new sectarian conflict in this deeply divided society.
General Sattler's assertions about routing the insurgency appeared optimistic, given the fact that Abdullah Janabi, the leader of Falluja's mujahedeen council, was still operating in the city. A recent Marine intelligence report also warned of the resurrection of the insurgency in the Falluja area should the American military reduce troop levels there, as has been planned.
Continuing skirmishes will slow the return of Falluja's civilians, many of whom fled before the fighting began. The decision to move people back is to be made by Prime Minister Ayad Allawi after recommendations by American commanders.
Almost all of the city has been heavily damaged, and the biggest question is how residents will react to seeing the vast swaths of destruction. Even before the devastation, residents were opposed to any American presence. American commanders say rebuilding efforts will win over the Fallujans, but attempts at reconstruction in other urban battle zones have stumbled badly.
Falluja is known as the City of Mosques, but the landscape is now dotted by broken minarets, many destroyed by airstrikes.
The interior minister, Falah al-Naqib, said at a news conference in Baghdad that families who fled would receive 150,000 dinars, or about $110, with their next monthly food ration. Engineers are evaluating how to restore power, water and sewage systems to the city, where hazards like downed power lines continue to pose a danger. .
What is thought to be the Zarqawi base was uncovered when Army and Iraqi soldiers returned Thursday to a compound where a fierce battle had unfolded Sunday, said Major Johnson, the Army officer. In the battle, he said, up to 25 insurgents armed with automatic rifles and rocket-propelled grenades had fought to the death from camouflaged positions.
"They fought well, better than the ones we'd seen," he said, noting that the fighters had mounted a coordinated defense and fired from deep in the house to hide their muzzle flashes.
By Thursday, the area had been reduced to a wasteland of rubble, rotting corpses, broken sewer lines and feral dogs. The house was in the southeast corner of a compound measuring about 4,300 square feet with a mosque at the center, Major Johnson said.
"That part of Falluja was the worst I've seen," he said. "There were bodies and body parts everywhere."
Videotape from a CNN cameraman who accompanied the American soldiers showed an Arabic sign on a wall with the words "Al Qaeda Organization." Inside, soldiers found vast piles of ammunition, Major Johnson said, including some that appeared to be Chinese and Jordanian, and a room with bomb-making equipment.
In Baghdad on Thursday, a suicide car bomb exploded next to an American military convoy, killing at least two Iraqis and wounding five. Later, American and Iraqi soldiers swept through the hostile neighborhood of Haifa Street, arresting more than 100 suspected insurgents, the Interior Ministry said.
In Mosul, the streets calmed from the uprising that took place last Thursday, when insurgents stormed a half-dozen police stations. Only 800 of the city's 4,000 policemen stayed on the job then, Brig. Gen. Carter Ham, the regional commander, told reporters on Thursday. At least 60 rebels were killed in clashes over the next 48 hours, and one American soldier and at least four Iraqi National Guardsmen died, he said.
[Reuters reported Friday that the Australian prime minister, John Howard, retracted his statement on Thursday that a body found in Falluja appeared to be that of the kidnapped Irish-born aid worker Margaret Hassan, 59.]
The Iraqi government was trying Thursday to piece together the whereabouts of 63 policemen from Karbala who may have been kidnapped this week as they returned from training in Jordan.
One of two trainees who escaped, Laith Saad Nema, said in an interview that the group was abducted at gunpoint from a hotel in Rutba, near the Jordanian border. He said foreign Arabs were among the kidnappers, and one captor said, "We will send their dead bodies to Allawi."
But the police chief of Karbala said in a televised interview that none of his men were missing.
In Basra, Mr. Mahdi, the spokesman for the Anger Brigades, said the new Shiite militant group was demanding that prominent Sunni clerics here and in Saudi Arabia issue an edict calling on Sunni extremists not to kill Shiites. Otherwise, he said, the Anger Brigades will start hunting down the Sunni fighters, believed to be foreigners.
In recent weeks groups of Shiites, many of them security officers, have been killed in an area around the insurgent strongholds of Mahmudiya and Latifiya, 20 to 30 miles south of Baghdad.
"Why are the Shiites being killed in cold blood?" Mr. Mahdi asked. "Some Arab countries want to create a civil war in Iraq. We are forming these brigades to resist those groups coming from outside the country."
Robert F. Worth reported from near Falluja for this article and Edward Wong from Baghdad. Reporting was contributed by Fakr Haider in Basra; an Iraqi employee of The New York Times in Karbala; and Richard A. Oppel Jr. in Mosul.