FALLUJAH, Iraq - American forces today pushed deeper into the last remaining insurgent stronghold in Fallujah, and the Iraqi government rushed reinforcements to Mosul, the country's third-largest city, where police lost control in the face of insurgent attacks.

Clashes erupted this afternoon on a major street in central Baghdad where insurgents often confront U.S. troops and Iraqi police, residents said. Gunfire could be heard in the area of Haifa Street, which runs north-south along the western side of the Tigris River, but it was unclear if American or Iraqi government forces were involved.

In a southern neighborhood, one U.S. soldier was killed and two others were wounded after being attacked with rocket-propelled grenades, small-arms fire and improvised explosive devices. An Iraqi interpreter also was injured.

Armed militants in Mosul attacked the main headquarters of a key Kurdish political party and assassinated a senior police officer as the governor asked for security forces to stabilize the situation.

Saadi Ahmed, an official with the Patriotic Union of Kurdistan, said an hour-long gunbattle broke out today between gunmen and the guards at the main headquarters. Guards killed six attackers and captured four others before the rest fled.

On Thursday, guerrillas attacked at least five police stations and political party offices there in what could be a bid to relieve pressure on their allies in Fallujah.

The unrest prompted the government to fire Mosul police chief Brig. Gen. Mohammed Kheiri Barhawi, according to deputy Gov. Khissrou Gouran. The move followed allegations by local officials that police abandoned their positions and in some cases cooperated with insurgents during Thursday's attacks.

Gov. Duraid Kashmoula said Mosul asked the Iraqi government for help in stabilizing the security situation.

"We asked the central government in Baghdad (for reinforcements) and God willing they should arrive today," he said. Kashmoula said he believed "there's infiltration among some (security) apparatuses from the saboteurs."

Four units from the Iraqi National Guard were ordered to Mosul from their bases near the Syrian border, Gouran said. The units consist of Kurds who used to be in the Kurdish peshmerga militia before being incorporated into the government's security force.

Gouran also said gunmen tried to storm a food distribution center in the Yarmouk area of Mosul 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.

The head of the city's anti-crime unit, police Brigadier Mowaffaq Mohammed Dahham, was assassinated by gunmen near his house, police officials said on condition of anonymity. His house in the Mithaq area then was burned.

Samarra's police chief, Taleb Shamel, told The Associated Press that he, too, was fired. There was no confirmation from the Iraqi government in Baghdad because offices were closed today, the Muslim day of rest and worship.

Shamel said no reason was given, but the city has been the scene of clashes between insurgents and the Americans since the coalition regained control from guerrillas in September.

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

As many as four dozen militants tried to break out toward the south and east late Thursday but were repelled by U.S. troops, the military said. U.S. forces also were positioned to the west near key bridges, blocking rebels from crossing the Euphrates River with patrol boats.

Troops have cut all roads and bridges leading out of the city and have turned back hundreds of men trying to flee during the assault. Only women, children and the elderly are being allowed to 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 one officer with the 1st Cavalry Division.

A U.S. soldier was killed Thursday night when his tank rolled over near Fallujah, the military said. Another American soldier was killed in northern Mosul during "combat operations," the military said.

Since the operation began four days ago, 18 American soldiers have been killed.

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

The current offensive was begun so the government could hold national elections in January, although Sunni clerics have called a boycott to protest the Fallujah operation. So far, the offensive has killed some 600 insurgents, 18 U.S. troops and five Iraqi soldiers, the U.S. military said. An additional 178 Americans and 34 Iraqi soldiers have been injured, the military said.

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 who were not actively fighting the Americans or assisting the insurgents have been killed.

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

Most of the insurgents still fighting in Fallujah are believed to have fallen back to southern districts ahead of advancing U.S. and Iraqi forces, although fierce clashes were reported in the city's west near the public market.

At a U.S. camp outside Fallujah, Maj. Gen. Richard Natonski, commander of the 1st Marine Division, said the operation was running "ahead of schedule," but he would not predict how many days of fighting lay ahead.

Militants have been using mosques as military strongpoints, he said.

"In almost ever single mosque in Fallujah, we have found an arms cache," Natonski said. "We have found IED-making (bomb-making) factories. We have found fortifications. We've been shot at by snipers from minarets."

Natonski also said he visited a "slaughterhouse" in the northern Jolan neighborhood where hostages were held and possibly killed by militants. He described a small room with no windows and just one door. He said he saw two thin mattresses, straw mats covered in blood and a wheelchair that apparently was used to transport captives.

U.S. officials believe the al-Qaida-linked terror movement of Abu Musab al-Zarqawi, who claimed responsibility for many of the kidnappings and beheadings of foreign hostages, used Fallujah as a base. They said they believe al-Zarqawi may have slipped away before the offensive.

Also, a Fox News reporter embedded with India Company of the 3rd Battalion, 5th Marine Regiment said the unit found five bodies in a locked house in northwest Fallujah on Wednesday. All the victims were shot in the back of the head. Their identities were not known, although there were indications they were civilians, the report said.

Late Thursday, Marines found the Syrian driver captured with two French journalists in August inside an undisclosed location in Fallujah. Capt. Ed Bitanga said the man told military officials he was separated from the journalists - Christian Chesnot, 37, with Radio France Internationale, and Georges Malbrunot, 41, with Le Figaro - about a month ago.

Their fate is unknown.

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Associated Press reporters Jim Krane near Fallujah and Tini Tran, Sameer N. Yacoub, Mariam Fam, Sabah Jerges, Katarina Kratovac and Maggie Michael in Baghdad contributed to this report.