U.S. warplanes and tanks pummeled Fallujah on Sunday as intense battles raged on the outskirts of the insurgent-held city.

Separately, a car bomb exploded late Sunday in Baghdad's Jadriya district, the Iraqi Interior Ministry said, according to the Associated Press. The ministry said the blast was aimed at an Iraqi police patrol and caused an undetermined number of casualties. Al-Arabiya television reported, however, that the explosion targeted a cafe, killing seven people, including some police officers, and wounding about 20.

In Sadr City, a sprawling Shiite slum in the capital, three Iraqis were killed when a mortar round hit a soccer stadium where a weapons buyback program was underway. The attack occurred minutes before the interim prime minister, Ayad Allawi, was scheduled to arrive.

In Latifiyah, a city held by insurgents about 25 miles south of Baghdad, nine Iraqi police officers were killed in an ambush Saturday, the Associated Press reported. The officers were returning from a U.S.-sponsored training course in Jordan.

The escalation of fighting in Fallujah came as hundreds of insurgents arrived from other cities for a long-anticipated offensive by U.S. forces, according to witnesses. The city was nearly empty except for the insurgents, who prayed in the streets and celebrated iftar, the evening meal that marks the end of each day's fast during the Muslim holy month of Ramadan.

The U.S. military has said its campaign in Fallujah is aimed at eradicating the network of Abu Musab Zarqawi, a Jordanian linked to al Qaeda. In an allusion to Fallujah, Iraq's national security adviser, Qasim Dawood, warned that the use of Iraqi cities as havens for terrorists was "something the government cannot accept or tolerate."

Representatives of the insurgents suspended negotiations with the government last week, saying the demand to turn over Zarqawi was unreasonable because he was not in Fallujah. Although some representatives indicated Sunday that they were open to further talks, the violence continued.

Witnesses reported that U.S. forces fired on a vehicle carrying a family fleeing the fighting, killing all five passengers. No casualty figures were provided by the U.S. military. A doctor at the Fallujah hospital said three Iraqis were killed in the fighting.

Details of the attack on the Iraqi police officers in Latifiyah were sketchy. Insurgents have targeted hundreds of police and National Guard recruits as part of their strategy to undermine the U.S.-led occupation. American officials have called the training of the new Iraqi security forces key to stabilizing the country and facilitating the withdrawal of U.S. troops.

The Iraqi government extended the weapons buyback program in Sadr City by two days in an effort to collect more arms. The program is part of a U.S. strategy to pacify areas held by insurgents in advance of nationwide elections planned for January. The agreement stipulates that loyalists of Moqtada Sadr, a rebellious Shiite cleric, exchange their weapons for cash and, in return, U.S. and Iraqi forces release detainees not convicted of any crimes. The program would be followed by as much as $500 million in reconstruction projects in the slum.

Despite the incentive, members of Sadr's militia, the Mahdi Army, have been slow to hand in weapons, U.S. military officials said.

Lt. Col. Florentino "Lopez" Carter, task force commander for the 2nd Battalion, 8th Infantry Regiment, 1st Cavalry Division, which patrols Sadr City, said in an interview that the effort had fallen "very short."

"I would say it certainly isn't a success," he said. "It doesn't send the right message that the Mahdi militia is focused on disarming and disbanding."

Carter said weapons have been handed over mostly by Sadr City residents not believed to be insurgents. "The only way we can measure it is based on number of weapons they hand over, and it just is nowhere near what we know to be the large weapons caches that they have," he said.

The U.S. military said in a statement Sunday that the program was "beginning to show a glimmer of success as more medium and heavy weapons are beginning to come in."

Carter said fighting in the slum has slowed considerably but that it was not clear whether the insurgents are using the pause to rearm and "re-seed" streets with roadside bombs, as occurred during a July cease-fire, or to disband.

Allawi was en route to the Sinaa soccer stadium when a mortar round struck it around 1 p.m., killing two Iraqi soldiers and an Iraqi civilian and wounding at least two others. It was unclear whether the attack was aimed at Allawi, who was scheduled to meet with members of Sadr's office and clerics at the stadium minutes later.

Iraqi guardsmen opened fire after the attack, shooting their automatic weapons randomly in response to reports of sniper fire. Two Iraqi police officers sitting in a nearby vehicle were wounded. Authorities immediately shut down the weapons handover program for the rest of the afternoon.

Allawi's convoy was diverted to a Sadr City government office building and his meetings with Sadr officials were postponed until later in the afternoon.

"I am very thrilled and pleased that things are moving in the right direction and arms are being surrendered to the Iraqi government," Allawi said after the meeting. "I call upon all Iraqi people throughout Iraq, whether in Basra, Nasiriyah, Fallujah, Ramadi or Mosul, to surrender their weapons and to respect the rule of law and to be part of the political process."

Large piles of weapons were visible at the stadium. They included hundreds of antitank mines, 14.5mm antiaircraft guns, 60mm, 82mm and 120mm mortar rounds, and artillery rounds.

Ahmed Saleh, 21, showed up at the stadium to turn in an old 60mm mortar launcher. Saleh, who said he was a member of the Mahdi Army, said the launcher was "not very good. I have another one at home that is much better."

Asked if he planned to hand it in, he said: "No, I'm going to keep it."

Special correspondents Bassam Sebti, Khalid Saffar and Omar Fekeiki contributed to this report.

Iraqi National Guardsmen try to control men lining up to submit weapons in Sadr City as part of a buyback program.Prime Minister Ayad Allawi, second from left, is welcomed by Shiite cleric Hussein Sadr during his visit to the sprawling Baghdad slum of Sadr City.