American troops in Iraq came across more challenges on Tuesday when US aircraft and tanks pounded targets in the besieged Iraqi city of Fallujah and killed 64 fighters loyal to radical Shi'ite cleric Moqtada al-Sadr near the holy city of Najaf.

Just hours after an American deadline expired for rebels in Falluja to hand over their heavy weapons, US troops blasted sections of the Golan district of the city, scene of heavy fighting between US Marines and Iraqi fighters on Monday.

The action, which lasted 30 minutes, was taken closely after anassault by US forces near Najaf.

Meanwhile, the United Nations special envoy on Iraq Lakhdar Brahimi, while speaking to the Security Council as the battle raged, urged the US-led coalition to bring the Falluja crisis to apeaceful end.

"The Coalition Provisional Authority is well aware that, unlessthis 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."

Reports quoted local doctors in Falluja as saying that hundredsof people have been killed in the US marines' siege of the town, ahotbed of uprising against the US-led forces, which began on April5 with the murder and mutilation of four American contractors there.

The killing came hours after Washington issued an ultimatum to al-Sadr to clear his militia and their arms from mosques there.

US officials said 57 Iraqis were killed in a single assault against a lone anti-aircraft gun spotted during clashes on the ground.

But a Sadr aide said only 19 of those killed were members of the militia.

It was the bloodiest encounter since Sadr and his army launcheda brief revolt against the US-led occupation three weeks ago before taking refuge in the city among Shi'ite Islam's holiest shrines.

Staff at two hospitals there said some of the casualties did not seem to be guerrillas.

Adding to the US burden, most Spanish troops in the occupation force who had been based in and near Najaf left Iraq on Tuesday ina withdrawal ordered by their new government in Madrid. US troops have had to replace the Spaniards in Najaf.

Washington is currently struggling to keep Iraq in order without inflaming anger at civilian casualties before the US authority hands sovereignty to an appointed Iraqi government on June 30.

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 greater risks for US efforts to win support among the Shi'ite majority.

US administrator Paul Bremer calls the situation there "explosive."

Sadr, 30, is wanted for the killing last year of another Shi'ite cleric. He has vowed to mount suicide attacks if the Americanstry to get him.

US forces and Iraqi militants clashed in the restive Iraqi cityof Fallujah from late Tuesday.

A US AC-130 gunship was firing on militant positions in the northern part of the Sunni Muslim city after three mortars fell ona US base as a two-week fragile truce appeared to be falling apart,witnesses were quoted as saying.

They added explosions shook Fallujah after dark and large plumes of smoke rose into the air as fighting erupted for the second straight night.

Blasts and gunfire went on steadily for more than half an hour in sustained fighting and flames could be seen rising from building while mosque loudspeakers in other parts of the city werecalling on firefighters to mobilize.

Source: Xinhua