The offensive appeared to be a new phase in the military strategy adopted last summer, when the American military commander in Iraq, Gen. George W. Casey Jr., took over with a plan to reclaim a string of cities that had fallen to insurgent control.

Between August and November, the strategy drove Shiite rebels out of the holy city of Najaf, forced a standdown by the same group in Baghdad's Sadr City district, and ended Sunni insurgents' stranglehold on Falluja, a major staging post for attacks.

The Falluja offensive ended with much of the city reduced to rubble, and insurgent groups still capable, weeks later, of mounting attacks from isolated pockets of resistance.

But American commanders acknowledged a more compelling reason that the offensive had proved less decisive than they had hoped. Many rebels fled ahead of the offensive, some north to Mosul, some southeast toward Sunni strongholds south of Baghdad, and others to Ramadi, 40 miles to the west, where insurgents last year took a measure of control almost on a par with their takeover of Falluja.

Ramadi, with a population of 400,000, is larger than Falluja and strategically as important. The stretch of the Euphrates involved in the offensive is a crucial communications corridor.

It has the main road and railway line connecting central Iraq to Syria, long accused by American commanders of acting as a sanctuary and staging post for insurgents and those financing them.

A major oil pipeline runs along the river's west bank. And to the east lies one of the country's most important power transmission lines, connecting a hydroelectric dam at Haditha to Baghdad.

The Marines said they had imposed a 10-hour night curfew around Ramadi and established "access control points" on roads into the city to screen vehicles "for terrorists and criminals, as well as weapons and materials" for making bombs. The command statement quoted Maj. Gen. Richard F. Natonski, commander of the First Marine Division, as saying the offensive had been requested by Iraq's interim government.

Three weeks after the elections, groups holding the principal blocs of seats in a new national assembly have yet to agree on the formation of a transitional government to guide the country through the next phase of an American-sponsored political blueprint.

The negotiations were set to resume Monday, after a two-day break for the Ashura religious festival.

Factions within the main Shiite political alliance are still contending over the competing candidacies of Ibrahim al-Jaafari, the nominee of an Iran-backed religious party, Dawa, and Ahmad Chalabi, a secular leader who was the Pentagon choice to lead Iraq before he fell out of favor after the American-led invasion.

The issue seems likely to be decided in a vote by the alliance's 140 members in the 275-seat national assembly, possibly this week. But formation of a government will require broader negotiations that could drag on for some time.

The interim prime minister, Ayad Allawi, a secular Shiite who controls his own party with 40 seats, said Sunday that he had not decided on his own future and hinted that he might accept a post in a coalition cabinet. Depending on the strategy adopted by the Shiite alliance, he said in an interview with Agence France-Presse, "I will decide if I can take part in the government or join the opposition."

The political maneuvering will take place in the shadow of a wave of suicide bombings during the Ashura festival, with at least 91 dead, that heightened Shiite leaders' concerns about an attempt by Sunni insurgents to foment sectarian strife and possibly even a civil war.

On Sunday the violence was more typical, with at least 11 people killed across the country.

The American command announced that a marine was killed in action on Saturday in Anbar Province, where Ramadi is the capital. The announcement did not say whether the marine had died in the new military offensive there.

Mostly, Shiite religious leaders have appealed for calm in the face of violence, saying Shiite interests are best assured through the political process. But a new note was struck Sunday in a rare pronouncement by Moktada al-Sadr, the young Shiite cleric who virtually disappeared after the second of two uprisings he led against American forces last year was crushed in August.

In an interview from an undisclosed location with Al Jazeera, the Arab satellite channel, he urged calm among Shiites, saying it was "wrong for a Muslim to kill a Muslim." But his codicil, blaming the United States and its coalition partners here for the bombings, sounded an ominous note for American commanders hoping that Mr. Sadr, who will have at least 12 assembly members under his control, has abandoned any instinct for fresh uprisings. By striking out against Sunnis for the bombing attacks, he said, Shiites would be "dragged into the plots of the West, which aim to destabilize the country and justify the presence of the occupation."