US aircraft and tanks blasted targets in the Iraqi city of Falluja on Tuesday, just hours after an American deadline expired for rebels to hand over their heavy weapons, witnesses said.
"I can hear more than 10 explosions a minute. Fires are lighting the night sky," one witness told Reuters as US forces pounded sections of the Golan district of the city, scene of heavy fighting between Marines and rebels on Monday.
"The earth is shaking under my feet," he said as live television pictures showed two large fires some 150 metres (yards) apart.
The action followed hard on the heels of an assault by US forces near the other Iraqi flashpoint city, Najaf, which spokesmen said killed dozens of fighters.
A US spokeswoman declined comment on what triggered the fighting in Falluja, which lasted about 30 minutes, or what the American target was in the city of 300,000 people.
Local doctors say hundreds of people have been killed in the Marine siege of the town, a hotbed of insurgency against the US-led forces, which began on April 5 following the murder and mutilation of four American contractors there.
In the holy city of Najaf, 64 fighters loyal to radical Shia cleric Moqtada al-Sadr were killed hours after Washington issued an ultimatum to him to clear his militia and their arms from mosques there, a US spokesman said.
Officials said 57 of them were wiped out in a single assault against a lone anti-aircraft gun spotted during clashes on the ground. A Sadr aide said only 19 of those killed were members of the militia.
It was the bloodiest encounter since the firebrand preacher and his Mehdi Army militia launched a brief revolt against the US-led occupation three weeks ago before taking refuge in the city among Islam's holiest Shia shrines.
Staff at two hospitals counted at least 23 dead and 34 wounded and said some of the casualties did not appear to be guerrillas.
At the funerals of five people killed, mourners chanted "Long live Sadr!" and slogans against the United States and its allies on Iraq's interim Governing Council.
SPANIARDS LEAVE: Adding to the US burden, most Spanish troops in the occupying force who had been based in and near Najaf, left Iraq on Tuesday in a withdrawal ordered by the new government in Madrid, where opposition to the occupation runs high.
US troops, who make up the vast bulk of the 150,000-strong force in Iraq, have had to replace the Spaniards in Najaf.
Not long before the fighting erupted in Falluja, Brigadier General Mark Kimmitt, the chief US military spokesman, said American forces were in no hurry to launch an all-out assault on the town.
Kimmitt said he was not sure any weapons were turned over on Tuesday, but that negotiations were "going well" and there were still plans for US troops to enter the town in joint patrols with Iraqi police.
Those had been planned to start as early as Tuesday, but did not. Kimmitt said it might now take another day or two, but it was unclear how the Marines expect to enter it without triggering new fighting.
Washington is struggling to douse these twin challenges to the new order in Baghdad without inflaming anger at civilian casualties before the US authority hands formal sovereignty back to an appointed Iraqi government on June 30.
Najaf, south of Baghdad, and Falluja, to the west, have provided acid tests among Iraq's two main Muslim communities.
The long oppressed Shia majority broadly welcomed the overthrow a year ago of Saddam Hussein, a Sunni.
But though few miss Saddam, impatience with disruption to daily life has angered many Iraqis, Shia as well as Sunni.
In Falluja, US commanders say they face up to 2,000 fighters - some diehard Saddam loyalists, others trying to reassert Sunni dominance of Iraq, and maybe about 200 foreign Islamic radicals, some possibly linked to al Qaeda.
Open warfare in Najaf would carry even greater risks for US efforts to win support among the Shia majority. US administrator Paul Bremer calls the situation there "explosive".
Sadr, a 30-year-old who draws authority from his late father, an ayatollah murdered by Saddam's forces, is wanted for the killing last year of another Shia cleric. He has vowed to mount suicide attacks if the Americans try to get him.