American forces are gearing up for a major operation in the insurgent bastion of Fallujah (search), where up to 5,000 Islamic militants, Saddam Hussein (search) loyalists and common criminals are hunkered down, U.S. officers said Friday.

U.S. planners believe many of Fallujah's 300,000 residents have already fled the city, which has become the symbol of Iraqi resistance and 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.

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 (search) 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 the showdown in Fallujah.

"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."

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.

Residents also reported hearing explosions Friday night near the industrial zone in the southeast part of the city.

Iraqi public outrage over reports of civilian casualties pressured the Marines into calling off their siege of Fallujah last April — a move which 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.

The April siege came after the killing of the four U.S. contractors.

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.

"Everybody knows that Abu Musab al-Zarqawi is another lie like the WMD," he told The Associated Press, referring to Saddam Hussein's supposed weapons of mass destruction. Hardline clerics who rule Fallujah insist al-Zarqawi is not in the city — a claim disputed by U.S. and Iraqi officials.

U.S. forces plan to use U.S.-trained Iraqi forces — especially around Fallujah's many mosques — to avoid allegations that the attack is nothing more than a bid by the Americans to crush a city widely known in Iraq for religious piety.

"The difference this time is it's driven by the" interim Iraqi government, Hejlik said. "They're calling the shots."

As a sign that preparations are under way, the first wave of troops from the Black Watch regiment arrived at the base near Baghdad, the British Defense Ministry said in London. The Americans asked the British to send troops to the area to free up U.S. forces for an assault on Sunni insurgents.

The rest of the 850-strong battle group from the Black Watch regiment were to arrive over the weekend, the ministry said. The base is 20 miles west of Mahmoudiya, a town that has seen frequent insurgent attacks, according to the report.

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.

"Just if you chop off the head, Zarqawi, it won't stop the insurgency," said Maj. James West, a Marine intelligence officer.

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

"Baghdad is the heart of Iraq and this is the throat to Baghdad," West said.

He estimated that between 50,000 and 60,000 civilians remain in Fallujah, which had an estimated population between 250,000 and 300,0000.

Fallujah residents confirm that up to half the population left at the start of Ramadan this month although some have returned.

Shupp, the Marine colonel, said insurgents are fortifying positions and are blocking roads with cement barriers and cars. Some Arab reporters who visited Fallujah in recent weeks say many neighborhoods on the edge of the city have been abandoned with only fighters remaining.

In other developments:

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

— Two car bombs exploded in the northern city of Mosul, killing an Iraqi civilian and slightly wounding five U.S. soldiers, U.S. and Iraqi 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.

— Aqil Hamid al-Adili, an assistant to the governor of Diyala province, was killed by gunmen as he was sitting in a friend's office, police said. Al-Adili had warned of insurgent infiltration in Iraqi forces after 50 U.S.-trained Iraqi soldiers were killed last weekend.

— An American contract worker from Columbus, Ga., was killed Wednesday in a car bomb attack, the Columbus Ledger-Enquirer reported. Travis Schnoor, 39, died when his vehicle flipped over after hitting an explosive device, the report said.