U.S. soldiers discovered a house in southern Fallujah on Thursday believed by U.S. military officials to be a main headquarters for the network of the Jordanian guerrilla leader Abu Musab Zarqawi, whose group has claimed responsibility for numerous bombings, kidnappings and beheadings across Iraq.

A black and white mural painted on a wall in the house, similar to banners shown in videos that have depicted the beheadings of foreign hostages, indicated that the house belonged to an "al Qaeda organization." Zarqawi has declared his allegiance to al Qaeda leader Osama bin Laden, and his group, initially called Monotheism and Jihad, recently adopted the name al Qaeda in Iraq.

In the house, the soldiers found documents that translators described as letters written by Zarqawi to his lieutenants, medical supplies from the U.S. Agency for International Development and boxes of ammunition from the Chinese and Jordanian armies.

Controlled by insurgents from late April until this month, when American and Iraqi forces mounted a massive offensive aimed at restoring government authority, Fallujah had become a hub for foreign guerrillas who joined Zarqawi's network, U.S. military officials have said.

Military officials said it was unclear when, if ever, Zarqawi had been in the house discovered Thursday in Fallujah's southern neighborhood of Shuhada. A U.S. intelligence source said Zarqawi apparently did not use Fallujah as his base of operations, and none of the leaders of the principal insurgent groups based in Fallujah -- Zarqawi, Abdullah Janabi and Omar Hadid -- were known to have been apprehended during the U.S. offensive there. Janabi had said he and Hadid remained inside Fallujah, and U.S. military officials suggested that Zarqawi might be in the northern city of Mosul.

Fighting persisted Thursday in Mosul and a string of other towns stretching across the region north and west of Baghdad known as the Sunni Triangle.

In Mosul, where 2,500 U.S. troops entered Tuesday, insurgents attacked the governor's office, killing a bodyguard, and mortars were fired at a U.S. base. Bombings in the northern towns of Kirkuk and Baiji killed six Iraqis, news agencies reported, and clashes erupted again in Ramadi, a provincial capital west of Fallujah. Fighting has surged in Ramadi since U.S. troops began their Fallujah offensive last week.

In Baghdad, where a car bomb near a U.S. convoy killed two people, U.S. and allied Iraqi forces swept through a restive swath of the capital that runs along Haifa Street. The Interior Ministry said 104 suspects were arrested, including foreigners.

Lt. Gen. John F. Sattler, commander of the 1st Marine Expeditionary Force, said Thursday that 51 U.S. troops and eight Iraqi soldiers had been killed in fighting in Fallujah and that 425 Americans and 40 Iraqi soldiers had been wounded. About 25 civilians were wounded and treated by U.S. military doctors, but no civilians were killed, Sattler said.

Marine and Army units continued on Thursday to clash with insurgents in Fallujah's Shuhada neighborhood. A Marine and an Iraqi soldier were killed in the fighting.

Uniformed, masked insurgents in Shuhada had attacked U.S. troops for several days with more than 15 rocket-propelled grenades, mortar rounds and sniper fire. U.S. warplanes and artillery subsequently bombed the area, and U.S. soldiers and Iraqi security forces returned there to look through the rubble.

The house said to have been used by the Zarqawi network, a simple concrete structure, was discovered Thursday on a block that Army Maj. David Johnson described as a "one-stop shop for terrorists."

"That part of town is the most dangerous place on Earth," said Johnson, a historian attached to 1st Infantry Division's Task Force 2-2, which conducted the raid.

Johnson said soldiers discovered at least nine bodies dressed in military fatigues, including that of a Sudanese man.

Iraqi security forces, acting as translators, identified letters written in black ink on white paper as correspondence between Zarqawi and his top aides. The letters reportedly contained requests for financing and weapons, Johnson said.

Soldiers hauled boxes filled with passports and identification cards out of the house. They also found bicycles and notes with instructions such as: "Go to the flour factory. There is something there for you."

In warehouses not far from the house, soldiers found a classroom with drawings of U.S. F-16 and F-18 fighter planes, a repair shop for anti-tank rounds and a factory for making car bombs where a Ford Explorer with a Texas registration sticker was parked. A garage with a roll-up door had been turned into a makeshift mosque.

Dead bodies were scattered among the rubble, and soldiers said they found no one alive.

A U.S. intelligence source said that while much of Zarqawi's organization was based in Fallujah, he apparently divided his time mainly between Baghdad and Ramadi. The source, a senior participant in the hunt for Zarqawi by U.S. and allied special forces and intelligence officers, spoke on condition of anonymity. The source said U.S. intelligence has been able to track Zarqawi occasionally, but never in time to move against him.

Sattler, the Marine commander, said the Fallujah offensive had "broken the back of the insurgency" in Iraq, disrupting rebel operations across the country.

"Each and every time we can force these individuals to go to new locations, expand their circle of friends -- if you want to call it that -- to include some that they don't know and they don't trust, they'll bring in rookies, more-junior people that will, in fact, make mistakes," he said. "This is going to make it very hard for them to operate."

But a veteran military analyst in Washington asserted that the United States arguably had suffered a political setback. "Fallujah has been a political victory for the insurgents," wrote Anthony Cordesman of the Center for Strategic and International Studies. "It has further polarized the Arab Sunnis, weakened Sunni participation in the interim government, and raised more questions about the independence and legitimacy of" interim Prime Minister Ayad Allawi's government.

Thaer Naqib, a government spokesman, acknowledged that the fighting in Fallujah has driven guerrillas elsewhere but insisted that their dispersal will aid U.S. and Iraqi efforts to subdue them ahead of nationwide elections planned for the end of January. "They have dispersed, but now that they have dispersed, we can finish them off as quickly as possible," he said at a news conference.

Naqib warned that Islamic clerics who incite violence would be seen as abetting terrorism. The statement echoed a similar vow by Allawi and followed a series of arrests of Muslim preachers in past weeks.

In particular, the government has targeted the Association of Muslim Scholars, which vocally supports the insurgency and has emerged as the most influential Sunni body. On Thursday, police also arrested Hashim Abu Raghif, the representative in Najaf for a militant Shiite Muslim faction loyal to the rebel cleric Moqtada Sadr, a spokesman for the group said.

In Baqubah, the U.S. military raided the office of the Iraqi Islamic Party, a Sunni group, according to the head of the office, Adnan Qaisi. This week, the U.S. military detained a high-ranking representative from the group's office in Baghdad.

A collection of 47 parties said it would boycott the election, citing the fighting in Fallujah and other cities. The group was dominated by Sunni factions, most prominently the Association of Muslim Scholars, but included a handful of Shiite groups.

"The battle of Fallujah has . . . has forced the parties to take a position," said Muthanna Dhari, the son of the association's leader. "The only way they can have elections is if the occupation forces announce the schedule of their withdrawal."

Correspondent Anthony Shadid in Baghdad and staff writers Barton Gellman in New York and Thomas E. Ricks in Washington contributed to this report.

A Marine handcuffs an Iraqi in Fallujah, where U.S. troops battled insurgents in the city's south.