As senior Iraqi officials declared Fallujah liberated, U.S. forces on Saturday continued intense combat operations aimed at securing the last section of the city from an insurgent force fighting with surprising discipline, organization and the trappings of a professional army, American commanders said.

In the southernmost section of Fallujah, where a showdown still loomed, U.S. soldiers discovered an underground bunker and steel-enforced tunnels connecting a ring of houses filled with weapons, medical supplies and bunk beds.

The fighters in the area were armed with assault rifles and rocket-propelled grenades, and dressed in blue camouflage uniforms with full military battle gear. U.S. soldiers reported finding American Meals Ready to Eat and other equipment that the U.S. government donated earlier this year to set up a local security force, which was quickly corrupted and taken over by insurgents.

The interim prime minister, Ayad Allawi, announced "a clear-cut victory over the insurgents and terrorists" in Fallujah but acknowledged that fighters had taken parts of the northern city of Mosul and had attacked sites in several other cities.

Commanders said the fighters in Fallujah exhibited far more skill on the battlefield than the ragtag insurgents who had fleetingly engaged U.S.-led security forces in the first days of the battle. U.S. military units reported heavy casualties for the second day in a row; 24 troops have been killed since the battle began.

"When we found those boys in that bunker with their equipment, it became a whole new ballgame," said Pfc. Troy Langley, 19, of Wister, Okla., who is assigned to Task Force 2-2 of the Army's 1st Infantry Division. "The way these guys fight is different than the insurgents."

The reality of the situation served to challenge the declarations of senior Iraqi officials, who as early as Friday were announcing that the battle for Fallujah was over in time for Iraqis to celebrate the end of Ramadan on Sunday in peace.

"It is with all pleasure that I announce to you that operation New Dawn has been concluded," the minister of state for national security, Qasim Dawood, said at a news conference in Baghdad, as Marine artillery and aerial gunships continued to pummel Fallujah 35 miles to the west. "Major operations have been brought to a conclusion."

U.S. soldiers and Marines, meanwhile, kept fighting.

"We control 90 percent, but the 10 percent that's left is the most difficult," said Capt. Erik Krivda, a member of Task Force 2-2 tactical operations command from Gaithersburg.

U.S. and Iraqi security forces have been battling fighters in this insurgent stronghold since ground troops followed a barrage of artillery fire into the city Monday night. Dawood said that more than 1,000 insurgents had been killed and 200 captured. A militia group, the Army of Mohammad, reported that 73 fighters had been killed.

It was unclear how many insurgents remained in the fight, or even the city. A U.S. military cordon around Fallujah proved porous, with Iraqi reporters entering the city from the south, and fighters leaving the same way. Others escaped by boat across the Euphrates River to the west, according to witnesses.

The insurgents who remained were very low on food, relying on fruit and canned goods, according to witnesses. But the fighters continued to harass U.S. forces, and the Iraqi troops who were trailing them, by moving through the maze of buildings behind the advance, and even answering American psychological warfare operations.

In areas controlled by U.S. forces, loudspeakers mounted on Humvees urged that "all fighters in Fallujah should surrender, and we guarantee they will not be killed or insulted."

From a loudspeaker on a mosque still controlled by insurgents, the fighters replied: "We ask the American soldiers to surrender and we guarantee that we will kill and torture them."

Dawood said the offensive failed to produce Abu Musab Zarqawi, the Jordanian leader of al Qaeda in Iraq, a guerrilla group that had made a base in Fallujah in an uneasy alliance with local insurgents. Indeed, by Friday morning, insurgent leaders claimed that 90 percent of the group's fighters had left the city and that the remaining 10 percent had been killed. Dozens of survivors were said to be traveling to Baghdad to carry out attacks.

An insurgent spokesman, speaking on al-Jazeera television, called on "scores or hundreds of brothers of the mujaheddin . . . to press the American forces" outside Fallujah. And a group of insurgents released a videotape to "announce the spread of the battle to all . . . parts of Iraq," according to Reuters news agency, which received the tape in Fallujah.

Large portions of Mosul remained under the control of insurgents. On the western side of the city of 1.8 million, residents reported no sign of government authority or a U.S. military presence. Police stations, overrun and looted by insurgents on Thursday and Friday, remained deserted. Streets were empty of all but rubbish and armed men who roamed the city. A car bomb detonated beside a convoy of Iraqi National Guard troops, injuring seven.

Dawood said, however, that Mosul was "not out of the control of the government."

"Just because a bunch of gangsters attacked police stations and declared that they were in control for not more than two hours does not mean that the government has lost control," he said.

Allawi said reinforcements of Iraqi security forces had begun arriving overnight to replace the Mosul police -- a force of perhaps 5,000 -- who largely deserted when the insurgents attacked.

Insurgents also harassed U.S. and Iraqi security forces in other cities in Iraq's Sunni Muslim heartland, including Baiji, Hawija, Tall Afar and Samarra.

In Baghdad, the Shiite Muslim mayor of the southern neighborhood of Bayaa, home to both Shiites and Sunnis, was assassinated, as was his Shiite predecessor earlier this year. Insurgents fired rocket-propelled grenades at the Education Ministry, the Associated Press reported.

Explosions echoed across the capital for a fourth day. Residents of the predominantly Sunni Adhamiyah neighborhood said insurgents fought U.S. troops for two hours overnight. Gunmen also roamed districts in the west and south overnight. On Saturday morning, a U.S. tank secured the southern edge of the Jadriya Bridge, which leads north toward the city center.

The battle for Fallujah, which insurgents have held since April, began with U.S. troops encountering only light to moderate resistance. Heavy artillery fire and air power seemed to knock the punch out of the resistance, which steadily pulled back from the main U.S. advance in the northern part of the city. But commanders warned that insurgents were likely to make a serious stand somewhere in the city.

Some U.S. commanders expressed surprise at finding fighters wearing uniforms and fighting like professionals. Unlike the insurgents who battled forces in the northern district, the forces in the south popped up, shot off a round and then moved.

But in urban combat, an organized force is also easier to identify, said Staff Sgt. Christopher Echevarria, 25, an Army Task Force 2-2 soldier from Crescent City, Calif.

"It's good they have uniforms," he said. "It makes it easier to know who to kill."

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

A U.S. Marine in the city of Fallujah provides cover for his comrades as they lead men arrested during a house search back to a U.S. camp.U.S. Marines call for support after coming under fire in Fallujah. Some insurgents were armed with assault rifles and rocket-propelled grenades.