The skies above Fallujah burned red late Monday as artillery, war planes and tanks pounded the Iraqi rebel bastion with a barrage of firepower at the start of an operation to retake the city.

Following a day of heavy shelling and missiles, U.S. Marines and soldiers stormed the northern entrance to the city west of Baghdad as plane and tank fire lit up the night sky.

“We are determined to clean Fallujah from the terrorists,” Prime Minister Iyad Allawi told a news conference in Baghdad, adding that the U.S.-led operation had his authority.

Intense fighting shook Fallujah in the morning. F-16 fighters screamed across cloudy skies, dropping bombs that sent up clouds of black smoke.

When air attacks eased, artillery shells rained down.

Between thunderous explosions, a cleric with a booming voice at a distant mosque rallied militants for what could be Iraq’s biggest battle since last year’s U.S.-led invasion.

“God is greatest, oh martyrs,” he said, telling fighters that waging holy war was an honor. “Rise up mujahideen.”

The Sunni Muslim Clerics Association urged Iraqi security forces not to fight with U.S. troops in Fallujah and “to beware of making the grave mistake of invading Iraqi cities under the banner of forces who respect no religion or human rights.”

A hospital doctor in Fallujah, Ahmed Ghanim, said 15 people had been killed and 20 wounded in the fighting.

Allawi earlier announced a stringent package of security measures to protect Iraq during the attempt to recapture the city, including a curfew in Fallujah and the closure of Baghdad’s international airport.

In earlier skirmishes, multinational forces seized a hospital and two bridges on the western edge of the city.

Clashes with the insurgents holed up in Fallujah were fierce, with a barrage of rocket, mortar and gunfire raining down as they tried to raise the new Iraqi flag above the hospital.

The Pentagon said U.S. forces seized the hospital first to provide medical care but also in the expectation that the presence of embedded reporters at the hospital would prevent inflated reporting of civilian casualties.

Allawi said 38 insurgents had been killed in the initial clashes and four foreign fighters had been detained, including two Moroccans.

The battle could prove the most intense since last year’s war to topple Saddam Hussein, with 2,000 to 2,500 fighters, some loyal to Iraq’s most wanted man Abu Musab al-Zarqawi, inside the city and prepared for brutal street fighting.

“The predictions are that they are going to stay and fight us here,” said Major Todd Desgrosseilliers. A combined force of some 10,000 American and 2,000 Iraqi troops are involved in the offensive.

The action came a day after Allawi declared a 60-day state of emergency across most of the country in a bid to control an escalating insurgency ahead of elections promised by January.

Rebels have transformed Fallujah into their fiefdom since a Marine assault in April ended in stalemate and left hundreds dead. It is estimated now that 80 to 90 percent of the city’s 300,000 inhabitants have fled.

As part of the security measures, Allawi said Iraq’s international airport would be closed to civilian flights for 48 hours and the country’s borders with both Jordan and Syria would be closed except for trucks carrying necessary food.

Allawi also declared that a curfew would be imposed on Fallujah and neighboring Ramadi along with other emergency measures.

With instability still rife elsewhere in the country, an American soldier was killed when gunmen fired on a military patrol in eastern Baghdad, the U.S. military said.

And at least three people were killed and 45 wounded when two suspected car bombs exploded within minutes of each other outside two Christian churches in southern Baghdad.

Meanwhile, at least eight Iraqis were killed and more than a dozen people wounded, including a U.S. soldier, in attacks Monday in central and northern Iraq, officials said.

In the restive Sunni city of Ramadi, at least four Iraqis were killed and one wounded in a car bomb attack as a US convoy was passing, said police.

Near the city of Samarra north of Baghdad, two Iraqi contractors working with the Americans were killed and two others were wounded when gunmen opened fire on their vehicle, a police spokesman said.