BAGHDAD, Iraq, April 5 — American officials on Monday stepped up their confrontation with Moktada al-Sadr, the militant Shiite cleric whose followers started an uprising in at least six Iraqi cities over the weekend, confirming that they had issued an arrest warrant for him but declining to say when they would execute it.
On Monday evening, American troops appeared to be moving into the area around Kufa, where Mr. Sadr's followers have seized control and the cleric has taken refuge in a heavily guarded mosque. Mr. Sadr shot back a defiant message, saying he would "welcome" a showdown with the American forces he has pledged to drive out of Iraq.
Also Monday, marines moved into Falluja, where four American security guards working for the occupation authority were killed in an ambush and then mutilated last week. In the Falluja operation, and the decision to arrest Mr. Sadr, American officials here appeared to have settled on a high-risk strategy, adopting a tougher stance instead of seeking, as they often have in the past, to avoid confrontation that might fan antagonism for the Americans.
American officials in Baghdad left little doubt that a decision had been made to try to seize Mr. Sadr, perhaps by issuing an ultimatum to militiamen who were milling around the mosque and guarding nearby rooftops in large numbers in Kufa, about 100 miles south of Baghdad. Reporters who entered Kufa on Monday said that militiamen at the mosque were armed with heavy machine guns and rocket-propelled grenades, which can destroy tanks, and that they had vowed to die for Mr. Sadr.
"You'll know," replied Brig. Gen. Mark Kimmitt, the spokesman for the American command, when asked when the bid to arrest Mr. Sadr would be made. "Yeah, let's just say there will be no advance warning," said Dan Senor, the spokesman for L. Paul Bremer III, the civilian administrator for Iraq.
Earlier, Mr. Bremer, addressing Iraq's newly established national security council, had called Mr. Sadr an outlaw and hinted that he would be taken down. "We will not tolerate this," Mr. Bremer said. "We will reassert the law and order that people expect."
The fighting, mostly quelled on Monday, had resulted in the deaths on Sunday of eight American soldiers in firefights in the Baghdad slum known as Sadr City, as well as a score of deaths among Sadr militiamen and Iraqi civilians spread across the fighting in central Baghdad, in Sadr City on Baghdad's northeastern outskirts, and in Kufa, Najaf and other cities, including the southern centers of Nasiriya, Amara and Basra.
American officials said the warrant for Mr. Sadr's arrest was issued in connection with the shooting and hacking to death of a rival Shiite cleric at a mosque in Najaf on the day after American troops captured Baghdad last April. But the announcement, months after the warrant was secretly approved, seemed timed to coincide with a new American military offensive — and to bolster American authority at a time when it has been jolted as never before in the 12-month occupation.
As American tanks and helicopter gunships patrolled Sadr strongholds in Baghdad on Monday, mopping up resistance in Sadr City and at Shuala, another Shiite slum, a separate offensive by American marines began at Falluja, 30 miles west of Baghdad. The objective in Falluja was to crush strongholds of Sunni Muslim insurgents held responsible for the ambush of the four Americans last Wednesday that produced some of the most grisly images of the war, mob violence in which the Americans' bodies were mutilated before two burned bodies were hanged from a bridge over the Euphrates River.
The Falluja action, too, had the earmarks of an American military command stung into using some of the toughest tactics at its disposal, beginning with a nighttime curfew, the encirclement of the city at dawn, earth barricades bulldozed across possible exit routes and, in one instance reported by The Associated Press, an Apache helicopter gunship firing a missile into a residential neighborhood.
Reporters reaching the city's outskirts from Baghdad were turned back at American checkpoints outside the city, precluding attempts to check the occupation authority's reports. General Kimmitt referred reporters to the First Marine Expeditionary Force, which assigned a 1,300-member force of marines and Iraqi security forces to the assault.
In the Falluja operation, and in the decision to arrest Mr. Sadr, the Americans appeared to have crossed a watershed. Faced with rampant insurgent attacks around Falluja, and with Mr. Sadr's relentlessly anti-American pronouncements, the Americans had chosen, often, to avoid direct clashes. The hope was that a rapid return to Iraqi sovereignty and a democratic government, coupled with American-financed reconstruction projects, would erode support for extremists, whereas harsh military actions might swell the pool of enemies.
But the shocks of recent days appear to have inspired a tougher stand. General Kimmitt said that position had been forced on the Americans.
"That decision was not independently made by us — the decision to go hard," he said. Rather, he said, it was made by the mob that mutilated the Americans in Falluja, and by attacks by Mr. Sadr's Mahdi Army on American troops fighting to regain control of government buildings in Sadr City on Sunday.
"Individuals who execute violence against persons inside of Iraq will be hunted down or captured or killed. It's that simple," the general said.
Mr. Senor, Mr. Bremer's spokesman, said the attacks on Americans aimed to derail the transfer of sovereignty to an Iraqi caretaker government on June 30, "and we absolutely will not tolerate that."
He added: "The debate in Iraq today is not between any two ethnic groups or any two regions. It's between moderates and extremists. And there's no room for extremists in Iraq, particularly when it comes to issues like the rule of law and justice and the idea that elections will determine who governs Iraq, not mob violence."
American officials appeared to struggle when they were asked if it was political expediency, not justice, that had led to the warrant for Mr. Sadr's arrest being prepared for execution now, at least six months after an Iraqi investigating judge drew up a list of 25 people, including Mr. Sadr, who were judged to have been involved in the slaying of Ayatollah Sayyed Abdul Majid al-Khoei.
The ayatollah was a prominent Shiite cleric whom the United States arranged to bring back to Iraq as American troops swept to victory over Saddam Hussein, in the hope of inserting a moderate, pro-Western figure into Najaf.
On April 10 last year, Ayatollah Khoei visited the Imam Ali mosque in Najaf for talks with the shrine's caretaker, only to be attacked and killed, along with the caretaker, by a mob shouting the praises of Mr. Sadr. The first 12 arrests in the case followed within a few months.
On Saturday, a cleric who was a close aide of Mr. Sadr's was arrested, precipitating the uprising. Then, on Monday, the occupation authority announced that Mr. Sadr was among the remaining 12 facing warrants.
Mr. Senor, Mr. Bremer's spokesman, said that the judge, facing a trial in the case within a month, had decided to "take another shot at trying to gather up individuals involved with the case," including Mr. Sadr, and implied that it was coincidental that this decision had come just as Mr. Sadr began his uprising.
But he acknowledged that the Americans were not altogether bystanders. "We've been working with this judge," he said, "and a number of these initiatives have been taken at his initiative, not ours."
Jeffrey Gettleman contributed reporting for this article from Kufa, Iraq.