Gen. John Abizaid, the American commander in the Middle East, also said in a news conference here that some troops would stay in Iraq longer than anticipated. He said that he had requested "two brigades of combat power, if not more," but did not say precisely which troops would delay their departure or for how long. The Army's First Armored Division had been scheduled to leave Iraq by the end of next month.
The disclosures came as American troops continued to withhold their firepower today outside three cities where insurgents have seized control and posed the most serious challenge yet to the year-old occupation. The cease-fire in Falluja, one of the three cities, has allowed Iraqi intermediaries time to try to broker diplomatic solutions to fighting there between Sunni Muslim insurgents and American troops.
"These are just initial discussions," said Lt. Gen. Ricardo S. Sanchez, commander of American forces in Iraq, who appeared with General Abizaid at the news conference. "We are not negotiating at this point until we achieve some confidence building and a period of stability. Then we would consider going into significant negotiations to end this battle."
Coalition forces have encircled the other two cities, the holy Shiite centers of Najaf and Karbala, south of Baghdad, in preparation for a possible strike to dislodge forces loyal to Mr. Sadr. United States officials warned that the resistance in all three centers would be crushed if the insurgents stalled too long.
Mr. Sadr is wanted by Iraqi and American officials in connection with the killing of a rival cleric a year ago, and American officials had said they were going to try to capture him. But General Abizaid said today that the Americans were also prepared to kill the radical cleric.
"The mission of U.S. forces is to kill or capture Moktada al-Sadr," the general declared. "That is our mission." He did not disclose whether American forces knew Mr. Sadr's whereabouts.
The seven American contractors, whose disappearance was confirmed today were in the same convoy as two American soldiers who were reported missing shortly after the ambush on Friday, General Sanchez said.
The military had reported the disappearance of the contractors, but had not said how many there were. General Sanchez said the contractors were in Iraq working for Kellogg Brown & Root, a subsidiary of Halliburton. They include Thomas Hamill of Mississippi, whom journalists photographed in a car shortly after his capture and who was later seen in a video made by his captors.
The military's new disclosure adds to the unspecified tally of foreigners — military and civilian — who have disappeared or been captured during clashes with Shiite and Sunni insurgents during the last week.
Seven Chinese civilian hostages captured on Sunday have been released, China's official Xinhua news agency reported today, but two Czech television journalists were reported missing today by their network, news agencies reported.
Failures among some Iraqi security forces have complicated the task of the American troops. General Abizaid said that video footage had showed Iraqi-trained policemen that had defected to Mr. Sadr's militia. "I think that these numbers are not large, but they are troubling to us," he said. "Clearly, we've got to work on the Iraqi security forces." In addition, he said that the inability of Iraqi security forces to "stand up to" Mr. Sadr's militias "was a great disappointment to us."
General Abizaid said that a plan to attach American "special operating forces" to Iraqi units would fortify the Iraqi security forces.
At Falluja, more than 1,300 marines, reinforced by United States Army and Iraqi troops, extended for a fourth day a halt to the offensive there that was intended to capture or kill the people responsible for the deaths two weeks ago of four American security guards working for the American military. The guards were ambushed and mutilated, with at least two of the charred bodies dragged through the streets and hung from a bridge.
Over the weekend, the fighting around Abu Ghraib, site of the most notorious prison of Saddam Hussein regime closed a strategic section of the six-lane expressway that runs westward from the capital, posing fresh supply problems for the marines encircling Falluja. American command spokesmen said that during the suspension of the offensive, the marines were firing only when fired upon.
Those American accounts were disputed by reports from the city, by families fleeing the fighting, by aid groups delivering food and medical supplies across the siege lines, and by Arab-language television channels like Al Jazeera. Those versions said American forces began new attacks on Sunday that had caused civilian casualties, including an airstrike said to have killed 11 people and wounded 50.
A hospital spokesman in Falluja, quoted by The Associated Press, said on Sunday the number of dead in the last week exceeded 600, including insurgents and civilians. Other accounts said more than 1,000 had been wounded. Some residents could be seen carrying bodies to a soccer field converted into a cemetery.
A spokesman for the American command, Brig. Gen. Mark Kimmitt, said American forces had no estimates of Iraqi casualties. But he rejected claims, some by Iraqi leaders sympathetic to the occupation, that the Marines had used tanks, infantry and air power disproportionately and indiscriminately in a way that had amounted to a collective punishment for Falluja's 200,000 people. "The forces out in Falluja have been tremendously precise, tremendously circumspect, and have not engaged in any violation of the laws of war," he said.
Marine casualties, including two killed in fighting at Falluja on Sunday and another who died of wounds received earlier in the day, have contributed heavily to the toll of at least 48 American servicemen killed in Iraq in the past week. That figure is the highest for any week since American forces captured Baghdad on April 9 last year.
Along with the killing of the American security guards — all veterans of elite American military units — the deaths among American forces have heightened the edginess among their commanders, and that showed in General Kimmitt's responses on Sunday.
When an Iraqi reporter at a news briefing said that General Kimmitt had spoken "of a clean war" in Falluja, while Iraqis watching Arab television channels like Al Jazeera, broadcast from the Persian Gulf state of Qatar, had an impression that "what is happening in Falluja is killing children," General Kimmitt responded, "Change the channel to a legitimate, authoritative, honest news station."
The battles of the last eight days, at the Sunni stronghold of Falluja, in Sunni and Shiite districts on the edge of Baghdad, and in at least six Shiite cities in the south, have raised anxious questions about the Americans' ability to maintain control in advance of the June 30 date set for Iraq to resume sovereignty under a transitional government.
A United Nations team that arrived here last week headed by Lakhdar Brahimi, a former Algerian foreign minister, appears to have made little headway so far in bridging wide differences among Iraqi political groups on how the transitional government should be formed, and on a still more contentious issue, an interim constitution with broad minority rights that has been rejected by Shiite leaders as a denial of Shiite majority rights. Even meeting with some of the key leaders has been a logistical nightmare.
How serious matters have become was evident across Baghdad on Sunday, with entire districts in this city of five million more deserted than they had been since the 21-day war last year. Bombings, drive-by shootings and the hostage-takings have convinced many Iraqis, and almost all foreigners, that any journey inside Baghdad — and certainly any journey outside the capital — is potentially lethal.
By nightfall Sunday, execution deadlines set in videotapes of the victims of two hostage-takings that were sent to Al Jazeera and another Arab satellite channel, Al Arabiya, had passed without any clear indication of the fate of those seized.
A message passed to Agence France-Presse on Sunday by an Iraqi claiming to speak for `the Iraqi resistance" said that one of three Japanese seized last week would be killed within 24 hours, and the others within 36 hours, if Japan failed to withdraw its 530-member military contingent.
Similarly, there was no word on the fate of Mr. Hamill, an employee of the Kellogg, Brown & Root division of Halliburton, who was seized by gunmen on a highway near Falluja on Friday. He was later videotaped in front of an Iraqi flag as a narrator said he would be killed by Saturday night if American troops were not withdrawn from Falluja. A British civilian contractor, Gary Teeley, 37, kidnapped in Nasiriya a week ago, was released and is safe with allied forces in southern Iraq, Britain's Foreign Office said.
General Kimmitt said one major flashpoint, the Shiite slum of Sadr City on Baghdad's northeastern outskirts, was "back in Iraqi control," with Iraqi police officers returning to stationhouses they had abandoned as the Sadr militiamen battled American tanks last Sunday. Similarly, he said, the southern cities of Kut, Diwaniya and Nasiriya had been mostly cleared of militiamen.
Only in Najaf and Karbala were the militiamen a significant force, he said, and American commanders had decided not to challenge them during the Shiite religious festival of Arbaeen, which wound down at sunset Sunday, with tens of thousands of pilgrims from across Iraq and neighboring countries, including Iran, heading home in long pedestrian files.
Kirk Semple contributed reporting from New York for this article.