The Iraqi government rushed reinforcements Friday to the country's third-largest city, Mosul (search), seeking to quell a deadly militant uprising that U.S. officials suspected may be in support of the resistance in Fallujah (search) — now said to be under 80 percent U.S. control.

Police in Mosul largely disappeared from the streets, residents reported, and gangs of armed men brandishing automatic weapons and rocket-propelled grenade launchers roamed the city, 225 miles north of Baghdad (search). 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.

Elsewhere, insurgents shot down a U.S. Army UH-60 Black Hawk helicopter near Taji, 12 miles 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.

In Fallujah, U.S. troops pushed insurgents into a narrow corner in the southern end of the city after a four-day assault that has claimed 22 American lives and wounded about 170 others. An estimated 600 insurgents have died, according to the military.

Despite the apparent success in Fallujah, violence flared elsewhere in the volatile Sunni Muslim areas, including Mosul, where attacks Thursday killed a U.S. soldier. Another soldier was killed in Baghdad as clashes erupted Friday in at least four neighborhoods of the capital. Clashes also broke out from Hawija and Tal Afar in the north to Samarra — where the police chief was also fired — and Ramadi in central Iraq.

The most serious incidents took place in Mosul, a city of about 1 million people, where fighting raged for a second day. Gunmen attacked the headquarters of the Patriotic Union of Kurdistan party in an hourlong battle that a party official said left six assailants dead.

Militants also assassinated the head of the city's anti-crime task force, Brig. Gen. Mowaffaq Mohammed Dahham, and set fire to his home.

"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."

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

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.

"We chose the path of armed jihad and say clearly that ridding Iraq of the occupation will not be done by ballots. Ayad Allawi's government ... represents the fundamentalist right-wing of the White House and not the Iraqi people," he continued — a reference to the interim Iraqi prime minister, who gave to the go-ahead for the Fallujah invasion.

In addition to firing the Mosul police chief, 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. Nevertheless, it appeared Iraqi authorities had no choice given the apparent failure of the city's police force to maintain order.

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 largest pocket of remaining resistance fighters were cornered Friday in the city's southwest as airstrikes and strafing runs continued.

"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.

Time magazine's Michael Ware, embedded with U.S. forces, said troops of the 2nd Battalion, 2nd Infantry Regiment who spearheaded the first push into the city early Monday found entire houses that were booby-trapped.

Fighting was so fierce that, on one occasion, U.S. troops fought insurgents room to room, just a few feet away from each other in the same house.

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 military says keeping men aged 15 to 55 from leaving is key to the mission's success.

"If they're not carrying a weapon, you can't tell who's who," said an officer with the 1st Cavalry Division.

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.

An audiotape purportedly made by Al Qaeda-linked terror suspect Abu Musab al-Zarqawi encouraged his fighters in Fallujah and said victory was near. He accused Kurds and Shiites in the Iraqi forces of abandoning their religion and said the offensive had been blessed by "the infidel's imam," Grand Ayatollah Ali al-Sistani, the leading Shiite cleric in Iraq.

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.

In Baghdad, Iraqi security forces, backed by U.S. troops, arrested one of those clerics, Sheik Mahdi al-Sumaidaei, and about two dozen other people after a raid of his Baghdad mosque uncovered weapons and photographs of recent attacks on American troops, U.S. and Iraqi officials said.

Mosul area deputy Gov. Khissrou Gouran said gunmen tried to storm a food distribution center in the city's Yarmouk area but were forced back by National Guardsmen and security guards. The gunmen were trying to destroy election registration cards held at the center, Gouran said.

In Washington, President Bush met with his top ally in the war, British Prime Minister Tony Blair, and warned that with Iraqi elections approaching, "the desperation of the killers will grow and the violence could escalate." But he said victory in Iraq would be a blow to terrorists everywhere.

Fallujah militants fought Marines to a standstill last April during a three-week siege, which the Bush administration called off amid public criticism over civilian casualties.

Many, if not most, of Fallujah's 200,000-300,000 residents fled the city before the assault. It is impossible to determine how many civilians not involved in the insurgency were killed.

Commanders said they believe 1,200-3,000 insurgents were holed up in Fallujah before the offensive.