"We are responding after being under fire. We are hitting enemy positions in the city since 6:30 pm. We are using aircraft and artillery fire," US Marine Spokesman Lieutenant Colonel T.V. Johnson said.
Aircraft and artillery pounded the southern Shuhada district and industrial zone of Fallujah, an AFP correspondent witnessed. Smoke mushroomed into the sky. War planes strafed the industrial zone. The general hospital had received undetermined number of casualties and panic swept the city, the reporter added.
One ambulance driver said he had taken two dead and 15 wounded to hospital, while families fleeing the air raids said there were more corpses and wounded trapped in the zone under attack.
Mosque loud speakers wailed "God is Great" amid the cacophony. "Structures used as fighting positions by anti-Iraqi forces were repeatedly struck by defensive counter-fires. Many of these buildings erupted in secondary explosions that are characteristic of hits on weapons/ammunition caches," the military said.
The US military has resorted to artillery and aerial assaults on alleged insurgent hideouts in Fallujah since the city became a no-go zone for troops after an April offensive ended with an agreement for Iraqis to police themselves.
13 US TROOPS KILLED: The Iraqi capital was steeped in blood on Tuesday as a fledgling truce in a Shia rebel bastion was shattered by running battles that officials said had resulted in scores dead, while 13 US soldiers were killed in the space of 24 hours.
Fierce clashes raged in Sadr City, an AFP correspondent said, with smoke billowing over parts of the over-populated Baghdad slum and jets roared above.
The health ministry said 40 Iraqis were killed and more than 270 wounded in overnight fighting between US forces and combatants loyal to radical Shia cleric Moqtada Sadr.
Sadr aide Sheikh Naim al-Qaabi said 15 Mehdi Army fighters were killed and 62 wounded.
"Last night was the most intense shelling of Sadr City since the Americans arrived in Iraq," he said, adding that heavy aircraft fire lasted from 11:00pm (1900 GMT) to 4:00am.
US army spokesman Lieutenant Colonel James Hutton reported several bomb and small arms attacks on US forces in Sadr City overnight and said one US soldier was killed in an ambush there on Tuesday.
A second US soldier was killed by small arms in western Baghdad, the military said.
The US military also reported the deaths of four other troops in separate attacks in the Baghdad area and one north of the capital on Monday, bringing the total number of soldiers killed to almost 1,000 since the March 2003 invasion.
The same day, the US military had suffered its worst single human loss in months when a car bomb ripped through a joint convoy, killing seven marines and three Iraqi national guards near Fallujah.
Suspected al Qaeda operative Abu Mussab al-Zarqawi's group claimed the bombing, as well as the downing of a drone flying over Fallujah, in a video obtained by AFP.