US forces in Iraq threw down a gauntlet to fighters from both main Muslim communities on Monday, threatening imminent assaults on two key towns if guerrillas do not accede to their demands.
US forces encircle both Najaf and Falluja, where Sunni insurgents fought a bloody new round in a three-week battle with US Marines. Eight guerrillas and a soldier died in clashes that damaged a mosque.
US and militia forces also fought near Najaf on Monday.
With the clock ticking down to a formal handover of power to Iraqis on June 30, the US-led occupation authority is racing to extinguish two serious challenges to its military control - while avoiding inflaming Iraqi public opinion.
US officials in Washington said President George W. Bush had asked commanders at Falluja to keep up negotiations. But they may start probing patrols into the city of 300,000 as early as Tuesday, risking serious confrontations.
Paul Bremer, the US administrator in Baghdad who called the situation in Najaf "explosive", issued an ultimatum to Shia cleric Moqtada al-Sadr to withdraw his Mehdi Army militia and its weapons from mosques and schools immediately.
Late on Monday, US forces clashed with Sadr's militiamen outside Najaf, south of Baghdad, leaving several Iraqi vehicles burning, witnesses said. There were no reports on casualties.
"The Coalition certainly will not tolerate this situation. The restoration of these holy places to calm places of worship must begin immediately," Bremer said in his written statement.
TUESDAY DEADLINE: In Sunni Falluja, long loyal to Saddam, US forces have also been largely holding back since launching a crackdown three weeks after the murder and mutilation of four American security guards. Local doctors say 600 people were killed. That has angered many fellow Arabs.
But US patrols, alongside Iraqi forces, would begin in Falluja "as early as tomorrow", Brigadier General Mark Kimmitt, a military spokesman for the authority, told a news conference.
Guerrillas, who may number up to 2,000 and include 200 or more foreign militants, have until Tuesday to hand in heavy weapons. After two weeks of a much abused cease-fire and Monday's fighting, which involved guerrilla rocket attacks and US air strikes, there seems only a slim chance of that.