US forces killed dozens of Iraqi fighters near Najaf overnight, hours after Washington issued an ultimatum to a radical cleric to clear his militia from mosques in the holy city.
The bloodiest clashes since Moqtada al-Sadr and his Mehdi Army began a revolt against the US-led occupation three weeks ago may mark a new phase in US efforts to dislodge him from Najaf, where he has taken refuge among some of the holiest shrines of Shia Islam.
Forty-three militiamen were killed, US officials said.
Locals said aircraft destroyed a militia checkpoint outside Kufa, 10 kilometres from Najaf, after a firefight. Staff at a nearby hospital counted at least six dead and 22 wounded. Some of the casualties did not appear to be guerrillas.
Some coalition allies are withdrawing troops as violence mounts and voters at home grow nervous.
Spanish troops left Najaf today, Spanish radio said. The loss of 1,400 Spaniards this month is forcing US commanders to fill the gaps with their own troops. The biggest other contingent, from Britain, may also be increased.
In a second flashpoint, the Sunni Muslim town of Falluja, local police took to the streets in force today as a deadline expired for guerrillas to hand in heavy weapons.
US troops had said they would join police on patrols as early as today, but Iraqi police said there would be further talks tomorrow on the joint patrols - which could reignite major fighting.