Defense Secretary Donald H. Rumsfeld declared yesterday that the military assault on the Iraqi city of Fallujah would be carried through to completion, unlike the U.S. Marine operation in April that was aborted after several days.

"I cannot imagine that it would stop without being completed," Rumsfeld said at a Pentagon news conference.

Pressed on the possibility that interim Iraqi Prime Minister Ayad Allawi might halt the offensive, Rumsfeld said he would be "amazed" if that happened. He said the Iraqi leader had been involved in extensive discussions on whether to proceed. "The decision to go included the decision to finish and to finish together," Rumsfeld said.

The U.S. military's previous attempt to clear Fallujah of insurgents has become a source of some contention within the Pentagon. The operation ended abruptly amid reports that hundreds of Iraqi civilians had been killed. Control was turned over to a security force made up of former Iraqi soldiers, who then failed to combat the militants.

Marine Lt. Gen. James Conway, who commanded the operation at the time, caused a stir in September when he told reporters that he had opposed the assault and the subsequent decision to withdraw from the city. Some senior U.S. authorities who served in Iraq have blamed the White House for the decisions both to attack and to withdraw.

Rumsfeld disputed that account yesterday, placing responsibility with U.S. military leaders in Baghdad, then led by Army Lt. Gen. Ricardo Sanchez, and with L. Paul Bremer, the chief U.S. administrator at the time. He noted that a number of members of the Iraqi Governing Council had opposed the operation, as had U.N. envoy Lakhdar Brahimi.

This time, the situation going into Fallujah is markedly different. Most notably, the Iraqi council is gone, replaced by Allawi and his interim government, which has appeared supportive of military action after trying unsuccessfully to strike a political deal with Fallujah representatives.

"That's the big difference between now and the last time that we did an operation in Fallujah -- is the fact that the Iraqis are in charge and they are trying very hard to pull all of the Iraqis into the political process," said Army Gen. George Casey, who has succeeded Sanchez as the top U.S. commander in Iraq.

Speaking by phone from Baghdad to journalists at the Pentagon, Casey predicted yesterday that the fight in Fallujah against an estimated 3,000 insurgents will be "a tough one." Although some militants had moved out of the city in anticipation of the offensive, he said, others had moved in.

"It's constantly changing, but I do believe that some of the key leaders will stay there and will fight with their soldiers," the general said.

He said the insurgents had lined the streets of Fallujah with car bombs and makeshift explosive devices as their "weapons of choice." They have erected "an outer crust" of defenses, he said, but are expected to retreat into the center of the city before making a last stand.

Although thousands of Fallujah's civilian residents have fled, as many as half -- or about 100,000 -- may still be in the city, Casey said. The U.S. Marine assault last spring was widely criticized for radicalizing an already restive city, prompting many residents to support the insurgents.

Asked about the danger of winning the battle for Fallujah but losing the war of public opinion, Rumsfeld expressed confidence that the discipline of U.S. troops would prevent "large numbers of civilians killed." He also noted that the state of emergency decree issued Sunday by Allawi's government instructed civilians on ways to reduce their exposure.

"Innocent civilians in that city have all the guidance they need as to how they can avoid getting into trouble," he said.

In an effort to offset some of the damage from the military assault, Iraqi authorities have drawn up plans to funnel humanitarian, medical and reconstruction support into Fallujah once the fighting stops, Casey said.

The roughly 2,500 Iraqi troops participating with the U.S. force of about 10,000 Marines and Army soldiers constitute a larger Iraqi contingent than took part in earlier military actions against insurgent strongholds in Najaf and Samarra. Their performance is being closely watched as a measure of an intensified U.S. program to train and equip Iraqi security forces.

Casey said he has received reports of some Iraqi troops "not making the movement to Fallujah." But he gave no numbers and attributed the no-shows to soldiers failing to return from regularly scheduled leaves. "It didn't have an impact on our plan," he said.

Although the Fallujah operation is aimed at eliminating what U.S. military officials have referred to repeatedly as the most troublesome insurgent safe haven in Iraq, Rumsfeld declined yesterday to call the offensive a final showdown, suggesting that squelching the insurgency is sure to involve more battles.

"I wouldn't use the word 'final,' " he said. "I think it's a tough business, and I think it's going to take time."