BAGHDAD, Iraq - Fearing an uprising in support of embattled insurgents in Fallujah, the Iraqi government rushed reinforcements to the key city of Mosul after police fled and gangs brandishing automatic weapons seized control of the streets.

The U.S. Army also diverted an infantry battalion from the fighting in Fallujah and sent them back to Mosul, U.S. military officials said Saturday.

The 1st Battalion, 5th Infantry Regiment, a unit of the 25th Infantry Division, was ordered back to Mosul late Thursday after militants attacked bridges, police stations and government buildings in the city, officials said.

The battalion, which is now part of the Stryker Brigade of Task Force Olympia, was already back in the Mosul area.

The surge of violence in Mosul, Iraq's third largest city, came as U.S. troops tightened control over most of Fallujah - cornering the largest pocket of remaining resistance fighters in the city's southwest as airstrikes and strafing runs continued.

U.S. forces said early Saturday that mortar fire from inside the city had nearly ceased after about three dozen bombing raids overnight. Two mosques were among the targets, the minarets of one of which were destroyed.

The four-day fight to wrest control of the city from insurgents and help pave the way for elections in January has claimed 22 American lives and wounded about 170 others. An estimated 600 insurgents have died, according to the military.

Elsewhere, insurgents shot down a U.S. Army UH-60 Black Hawk helicopter north of Baghdad, wounding three crew members, the military said. It was the third downed helicopter this week after two Marine Super Cobras succumbed to ground fire in the Fallujah operation.

Four other U.S. helicopters were hit by groundfire in two separate attacks near Fallujah but their crews were not hurt and were able to return to base.

Despite the apparent success in Fallujah where the Americans are said to control 80 percent of the city, violence flared elsewhere in the volatile Sunni Muslim areas, including Mosul, a city of about 1 million people.

Residents reported that police largely disappeared from Mosul, 225 miles north of Baghdad, as gunmen carried out several actions, including an attack on the headquarters of the Patriotic Union of Kurdistan party and the assassination of the head of the city's anti-crime task force.

On Saturday, residents reported relative calm as Iraqi troops patrolled in parts of the city. The U.S. military said 10 Iraqi troops have been killed since clashes erupted late Thursday.

Capt. Angela Bowman, a spokeswoman at the U.S. Mosul headquarters, said "some of these attacks are in support of the resistance in Fallujah."

Responding to the crisis, Iraqi authorities dismissed Mosul's police chief after local officials reported that officers were abandoning their stations to militants without firing a shot.

Iraqi authorities also dispatched four battalions of the Iraqi National Guard from garrisons along the Syrian and Iranian borders.

Most of the reinforcements are ethnic Kurds who fought alongside American forces during the 2003 invasion - a move which could inflame ethnic rivalries with Mosul's Sunni Arab population.

"With the start of operations in Fallujah a few days ago, we expected that there would be some reaction here in Mosul," Brig. Gen. Carter Ham, commander of U.S. forces in the city, told CNN from Mosul.

Ham said he doubted the Mosul attackers were insurgents who fled Fallujah and said most "were from the northern part of Iraq, in and around Mosul and the Tigris River valley that's south of the city."

In a telephone interview with Al-Jazeera television, Saif al-Deen al-Baghdadi, an official of the insurgents' political office, urged militants to fight U.S. forces outside Fallujah.

"I call upon the scores or hundreds of the brothers from the mujahedeen ... to press the American forces outside" Fallujah, al-Baghdadi said.

At a U.S. camp near Fallujah, Lt. Gen. John Sattler, commander of the 1st Marine Expeditionary Force, said U.S. and Iraqi forces now occupy about 80 percent of the city, and that clearing operations are continuing to find caches of weapons and ammunition.

Army and Marine units moved to tighten their security cordon around Fallujah, backed by FA-18s and AC-130 gunships.

"The rout is on," said a 1st Cavalry Division officer. "It won't be long now."

Iraqi forces were charged with searching every building in Fallujah, working from north to south, the military said.

In the city's north, U.S. forces reported roving squads of three to five militants shooting small-arms fire and moving easily through narrow alleyways. Troops were finding numerous weapons caches, the military said.

Troops have cut off all roads and bridges leading out of Fallujah and have turned back hundreds of men trying to flee the city during the assault. Only women, children and the elderly can leave.

The Fallujah operation threatens to enflame passions within the Sunni community, not only against the American presence but against the Shiite majority, whose clerical leaders have by and large remained silent over the killings of Muslims in the city.

U.S. and Iraqi authorities launched the Fallujah operation to restore government control so that national elections can go ahead by the end of January as planned. However, hardline Sunni clerics are calling for a boycott to protest the Fallujah attacks.