U.S. and Iraqi troops entered Mosul in force Tuesday to retake streets and police stations seized by fighters in the northern city last week, while a prominent Iraqi insurgent claimed that the battle in Fallujah was only the beginning of an uprising that has already roiled parts of Iraq dominated by Sunni Muslims.

"The Americans have opened the gates of hell," Abdullah Janabi said Monday in Fallujah, a city U.S. commanders have said they now control after a week of often fierce fighting. "The battle of Fallujah is the beginning of other battles."

Iraqi officials had said they believed Janabi, a 53-year-old Sunni cleric, had fled the city before U.S. troops pushed into the insurgent stronghold. But he spoke from the city's southern section, at times boasting of losses inflicted on U.S. troops and at other times insisting that other insurgent leaders remained in Fallujah with him.

After fighting erupted in Fallujah last week, insurgents moved to open a second front in Mosul, seizing control of parts of the city and attacking bridges and six police stations. Some stations were looted of body armor, uniforms, weapons and radios, and at least three were too damaged to be reoccupied.

On Tuesday, more than 2,500 U.S. troops entered Mosul, where gunfire echoed through rain-soaked streets that were largely deserted on the last day of a three-day Muslim holiday. The city's five bridges across the Tigris River were closed, and a curfew was imposed from 4 p.m. to 6 a.m. The troops met little resistance, although four U.S. soldiers were wounded by a car bomb that detonated near their convoy on the city's western edge, said Lt. Col. Paul Hastings, a spokesman for U.S. forces in Mosul.

When the fighting first flared in Mosul, many members of the city's 5,000-man police force fled, and the police chief, Brig. Gen. Mohammed Kheiri Barhawi, was fired following complaints that some officers had cooperated with the insurgents. Hastings said about 1,000 policemen had returned, but he acknowledged that reconstituting the units posed a "huge challenge."

The clashes that have erupted north and west of Baghdad since last week constitute the most intense fighting since the insurgency began in earnest six months ago. The U.S. military has reported 130 to 140 attacks a day, including car bombings, roadside mine blasts and ambushes, along with sabotage and intimidation of Iraqi security forces. On Monday, when fighting broke out in several northern and western cities, seven car bombs were detonated -- two in Mosul and five in the region around Fallujah.

The daily tally is comparable to that seen in a bout of fighting in Fallujah and southern Iraq in April, but across a far smaller area.

Residents reported renewed fighting Tuesday in the northern towns of Baiji and Baqubah. In Balad, about 40 miles north of Baghdad, an improvised mine detonated near a convoy, killing a U.S. soldier and injuring another, the military said.

Since declaring Fallujah liberated on Sunday, U.S. commanders have played down the continuing battles there, but bursts of gun and mortar fire exploded Tuesday across the battle-scarred city as American forces continued to pursue insurgents. Shooting could be heard for most of the afternoon on the city's northern edge, where the U.S. military estimated about 100 fighters were still operating in neighborhoods that troops first entered a week ago.

Organized bands of fighters clashed with U.S. troops Tuesday on Fallujah's southern outskirts. Black smoke rose from burning rubble after U.S. artillery batteries fired 155mm rounds into suspected insurgent hideouts.

The U.S. military said it had killed at least 1,200 insurgents and detained hundreds in fighting that has destroyed scores of buildings in the conservative, deeply religious city. At least 38 U.S. troops and six Iraqi soldiers have been killed, the most in a single offensive since the fall of Saddam Hussein on April 9, 2003.

"I think it was a very substantial victory," Army Gen. George W. Casey Jr., the U.S. military commander in Iraq, said after touring Fallujah on Tuesday.

"Fallujah is no longer a terrorist safe haven," Casey said. "That's a major accomplishment with the Iraqi security forces and for the coalition forces, and it's a major way ahead for Iraq."

However, Janabi, the insurgent leader, said Monday: "We still have our strength, our force and ammunition, and the battle is long, very long. And we will turn Iraq into one big Fallujah."

"It is only the beginning, from a military point of view," said Janabi, who heads the mujaheddin shura, an 18-member council of clerics, tribal sheiks and former Baath Party members that assumed control of the city of 250,000 shortly after Marines aborted their first attempt to capture it in April. "We have succeeded in drawing them into the quagmire of Fallujah, into the alleys and small pathways. They have fallen into the trap of explosive charges, land mines and, now, the defenders' short supply lines inside the neighborhoods."

Speaking in an undamaged house in the city's Nazal district, Janabi was protected by several bodyguards and wore an explosive vest, the wires that would detonate it dangling a few inches apart. A bodyguard said the cleric preferred martyrdom to "dying on his bed like a camel."

"The cause will not die if the individuals die," Janabi said. "It will survive until the last Iraqi holy warrior dies or runs out of bullets." He added, "If the military leaders agree on another area where we will inflict more losses on them, then we will."

Janabi mocked the statement of a senior Iraqi official who on Saturday told reporters that Janabi and another insurgent leader, Omar Hadid, were "cowards" who had fled the city before the offensive. Hadid is a ranking figure in the group now known as al Qaeda in Iraq, headed by the Jordanian militant Abu Musab Zarqawi, who by all accounts left Fallujah weeks before the U.S. offensive began on Nov. 8.

"I am here," Janabi said. "You can see me. And if you wait for a while, you can see Omar Hadid. He is still in the city."

The Iraqi government announced the capture of another insurgent leader, Moyad Ahmed Yaseen; news of his detention had been broadcast over a Marine public address system earlier in the day. Yaseen was military leader of the First Mohammed Army, the largest militia in Fallujah, which claimed a membership of 6,000 mostly Iraqi volunteers. In announcing the capture, Ayad Allawi, Iraq's interim prime minister, said the group "killed a number of Iraqis, Arabs and foreigners in Iraq by beheadings."

"We arrested the whole leadership," he said.

Fearful of a backlash, Allawi and other Iraqi officials have dismissed suggestions of a humanitarian crisis in Fallujah, where disputed reports of civilian casualties in April unleashed anger across Iraq. They have said most families fled the city before the American assault, an assertion confirmed by some residents. Most reporting in Fallujah is limited to journalists embedded with the U.S. military.

"The Iraqi government strongly rejects suggestions from some sources that there are shortages of supplies in Fallujah," a statement from Allawi's office said.

A spokesman, Thaer Naqib, said 12 trucks carrying food, water and medical supplies entered the city Tuesday under an escort of Iraqi troops.

But Amnesty International, in a release Tuesday, said the city still lacked water, electricity and organized means for evacuating the wounded. The Iraqi Red Crescent Society has been barred from delivering relief supplies, and doctors in Fallujah complained that U.S. troops were preventing them from moving in the city to treat wounded.

Wary of the role played by mosques and Islamic parties in the April uprisings, U.S. forces and the Iraqi government have cracked down on some religious activists, suggesting that they are abetting the insurgency. On Tuesday, U.S. forces detained Naseer Ayaef, a leader of the Iraqi Islamic Party, in a pre-dawn raid on his home, said Ayad Samarrai, a spokesman for the party, which has taken part in the U.S.-led political process.

Staff writer Jackie Spinner in Fallujah and special correspondents Naseer Nouri and Bassam Sebti in Baghdad contributed to this report.