A new US bombardment struck the same area of the besieged Iraqi city of Falluja today which took an overnight pounding from the air, local residents said.
"There are explosions in the Golan district. It seems to be shelling. There is also heavy machinegun fire coming from the American positions," one witness said.
US warplanes were flying over the Sunni Muslim city west of Baghdad but were not attacking, he added.
US aircraft and tanks pounded targets in the city last night, just hours after an American deadline expired for rebels to hand over their heavy weapons, witnesses say.
The bombardment followed an assault by US forces near the other Iraqi flashpoint city, Najaf, which spokesmen said killed dozens of fighters.
A terse US statement said the fighting in Falluja, a city of 300,000 people, began with an attack on marine positions.
"Marines responded by directing precision weaponry against enemy forces in order to defend themselves," it said without giving any further details.
UN special envoy to Iraq Mr Lakhdar Brahimi, speaking to the UN security council as the battle raged, urged the US-led administration to bring the Falluja crisis to a peaceful end.
"The coalition provisional authority is well aware that, unless this standoff is brought to a resolution through peaceful means, there is great risk of a very bloody confrontation," he said.
"They know as well as - indeed, better than everyone else - that the consequences of such bloodshed could be dramatic and long-lasting."
Local doctors say hundreds of people have been killed in the marines' siege of the town, a hotbed of insurgency against the US-led forces, which began on April 5th following the murder and mutilation of four American contractors there.
Near 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.
Officials said 57 were killed
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 launched a brief revolt against the US-led occupation three weeks ago before taking refuge in the city among Shia Islam's holiest shrines.
Staff at two hospitals counted at least 23 dead and 34 wounded and said some of them did not seem 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.
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. U.S. troops have had to replace the Spaniards in Najaf.
A Spanish government source said Spain would discuss with anti-war allies Germany and France on Wednesday finding a solution to the conflict in Iraq within the United Nations framework.