U.S. forces launched airstrikes against suspected militant bases in Fallujah (search) and carried out probing attacks on the city's outskirts, as they prepared for a major operation in the insurgent bastion that has become the symbol of Iraqi resistance.

U.S. planners believe many of Fallujah's 300,000 residents have already fled the city, where militants last spring ambushed and killed four American contractors, mutilated their bodies and hung them from a bridge.

U.S. and Iraqi authorities want to curb the increasingly violent Sunni Muslim insurgency in order to hold nationwide elections by Jan. 31. Up to 5,000 Islamic militants, Saddam Hussein loyalists and common criminals are hunkered down in Fallujah, U.S. officers said Friday.

American officials stress that the final order to launch a big operation would come from Iraqi Prime Minister Ayad Allawi (search), who has warned Fallujah to hand over followers of terror mastermind Abu Musab al-Zarqawi or face attack.

Allawi has issued no such order, but preparations are clearly under way, including the movement of British soldiers into areas close to Baghdad so that American forces can be redeployed for a showdown.

"We're gearing up to do an operation and when were told to go we'll go," Brig. Gen. Dennis Hejlik, deputy commander of the 1st Marine Expeditionary Force, said at a camp near Fallujah. "When we do go, we'll whack them."

Insurgents have stepped up attacks across Iraq by 25 percent since the start of the holy month of Ramadan two weeks ago, mainly targeting Iraqi security forces.

Witnesses said Saturday that Iraqi forces opened fire on six vehicles, including three minibuses, after a U.S. convoy came under attack in a central Iraqi town south of Baghdad, killing or injuring more than a dozen people.

Witnesses speaking to Associated Press Television News said an American convoy was attacked early Saturday near the town of Haswa, about 25 miles south of the capital.

After the U.S. troops pulled out, Iraqi police and National Guards arrived on the scene. Witnesses said Iraqi troops opened fire randomly and used hand grenades, hitting three minibuses and three trucks.

The U.S. military had no immediate response.

Doctor Salah al-Janabi of Iskandariyah General Hospital said the hospital received at least 11 bodies, and at least another 15 people were injured. He said more casualties could have been taken to other hospitals.

On Saturday, gunmen fired on a police convoy just outside Baghad, causing one of the vehicles to burst into flames, police said. Witnesses said they saw three policemen trapped inside the burning vehicle, but officials did not give a casualty toll.

Also, a U.S. military patrol came under gunfire when it stopped to aid Iraqis hit by a roadside bomb, the U.S. command said Saturday.

The bomb detonated Friday evening, injuring three civilians, and a U.S. patrol in the area stopped to give aid and came under attack by small arms fire, the U.S. military said. Two more Iraqi civilians were wounded in the firefight. No American soldiers were wounded.

Meanwhile, Marines have been hitting Fallujah with frequent airstrikes, targeting buildings believed used by al-Zarqawi's followers. Marines have also launched probing attacks into Fallujah's outskirts to test insurgent defenses, Marine Col. Mike Shupp said.

A U.S. warplane fired at a house in the eastern Askari district of Fallujah around sundown Friday, witnesses said. Firefighter Salam Hameed said five bodies were pulled from under the rubble. Another four people were injured.

Iraqi public outrage over reports of civilian casualties pressured the Marines into calling off their planned siege of Fallujah last April — a move that strengthened the insurgents' hold on the Sunni city 40 miles west of Baghdad and likely contributed to the dramatic deterioration of security in the capital itself.

On Friday, a Sunni cleric in Baghdad, Sheik Mahdi al-Sumaidaei, warned the Americans and Iraqis against launching a full-scale attack on Fallujah. If they do, he said Sunni clerics in the capital will issue a fatwa, or a binding religious decree, ordering Muslims to launch street protests and a campaign of civil disobedience.

The United States has offered a $25 million reward for the capture of al-Zarqawi, a Jordanian extremist who announced his allegiance to Al Qaeda on the Internet this month. Al-Zarqawi's movement is responsible for numerous car bombings and beheadings of foreign hostages, including three Americans.

However, U.S. officials acknowledge that even if they kill or capture al-Zarqawi, the insurgency is likely to continue.

Nevertheless, re-establishing control in Fallujah would cut vital links among insurgent groups and affect their ability to plan and carry out attacks, particularly in Baghdad.

In other developments:

_ The Bangladesh government confirmed Saturday that one of its nationals was taken hostage by militants in Iraq, officials said. Abul Kashem, 42, a truck driver who works for a Kuwaiti transport company, was abducted near a U.S. military base in Iraq last week, officials said.

_ U.S. troops detained a Croat truck driver in Iraq. The U.S. Embassy in Croatia said Damir Mikulic was being held at Camp Bucca, near Umm Qasr in southern Iraq, for filming U.S. military bases and training exercises. His video camera allegedly held more than eight hours of sensitive footage.

_ Kidnappers released a 7-year-old Lebanese boy a week after they grabbed him as he was walking home from school.