The commander, Gen. John P. Abizaid, pledged that the United States and its allies would not be driven from Iraq by the guerrilla attacks, which today killed one American soldier and wounded at least six others around Baghdad. But he cautioned that pacifying Iraq might require fresh American troops to spend yearlong tours there, double the normal duration of Army forces on peacekeeping duty.

The assessment of Iraqi resistance by General Abizaid was a significant change from previous comments by senior Pentagon officials, including Defense Secretary Donald H. Rumsfeld, who has said that the insurgents' raids were too haphazard to qualify as a guerrilla war or organized resistance.

"I believe there's midlevel Baathist, Iraqi intelligence service people, Special Security Organization people, Special Republican Guard people that have organized at the regional level in cellular structure and are conducting what I would describe as a classical guerrilla-type campaign against us," General Abizaid said.

"It's low-intensity conflict, in our doctrinal terms, but it's war, however you describe it," he added during his first news conference since being sworn in last week as the Central Command's senior officer.

Pentagon planners disclosed today that a number of new or unusual options are under consideration to replace battle-weary American ground forces in Iraq like the Army's Third Infantry Division and the First Marine Expeditionary Force.

The Army's first Stryker Brigade Combat Team, built around a new, lightly armored vehicle named Stryker, might be ordered to Iraq this fall, and Pentagon officials are analyzing whether to activate, early next year, any of the National Guard's enhanced brigades, which are specially designated units that train with the active-duty Army and are assigned its most modern combat equipment.

Pentagon officials said other options included assigning the Marine Corps a major piece of the long-term peacekeeping operation — though it has traditionally been an expeditionary force that seizes territory but does not hold it for lengthy periods — or turning to individual Army battalions or brigades if they have not yet seen duty in Iraq.

The focus on assigning marines to Iraq peacekeeping duties — as well as pressing allies for contributions of forces — is driven at least in part by the fact that of the Army's 33 active-duty combat brigades, 21 already are deployed: 16 in Iraq, two in Afghanistan, two in South Korea and one in the Balkans, a Pentagon official said today.

In an effort to rally allies to contribute forces for the stabilization mission in Iraq, Secretary of State Colin L. Powell said today that he was discussing with his foreign counterparts and the United Nations secretary general, Kofi Annan, the possibility of introducing new United Nations resolutions that might make it possible for countries like India to take part in the coalition in Iraq.

India, as well as France and Germany, has said it will send troops only under United Nations auspices.

"I've had some discussions with other ministers, as well as with Secretary General Annan, whether or not it would be appropriate to start discussions about other U.N. resolutions," Mr. Powell said. "But that's as far as these preliminary discussions have gone." 

At the United Nations, Mr. Annan spoke of the Security Council's efforts on this issue, saying, "I am sure, if there is will, they will find the language to broaden and internationalize the process."

Stress on American ground combat units has been evident in recent days as members of the Third Infantry Division, the longest-serving Army unit in Iraq, were quoted in television and other interviews as being openly critical of Mr. Rumsfeld and their mission after hearing that the promised return home of their final two brigades might slip into late autumn. One brigade has begun its journey home. 

General Abizaid, himself an Army officer, complimented the combat prowess and courage of the division, and pledged again that his commanders would try to send the troops home by September — but showed no patience for public criticism of Mr. Rumsfeld by men and women in uniform.

"None of us that wear this uniform are free to say anything disparaging about the secretary of defense or the president of the United States," General Abizaid said during his Pentagon briefing. "We're not free to do that. It's our professional code. Whatever action may be taken, whether it's a verbal reprimand or something more stringent is up to the commanders on the scene and it's not for me to comment."

American troops in Iraq today or those to be deployed should expect more attacks — not only from Iraq guerrillas, but from foreign terrorists as well, the general said.

General Abizaid said, "It's unclear, but it's troubling, that Al Qaeda either look-alikes or Al Qaeda people are making an opportunity to move against us." The Ansar al-Islam terrorist group, whose camps in northeastern Iraq were attacked during the war, is also trying to reconstitute within Iraq, and foreign money is underwriting some of these terrorist efforts, General Abizaid said.

Of the anti-American forces operating in the central and north-central parts of Iraq, where American forces have come under heaviest attack, General Abizaid, "They're better coordinated now."

The insurgents, he said, are showing "some level of regional command-and-control" that indicates planning beyond individual small groups striking only at targets of opportunity.

General Abizaid refused to be drawn into discussing whether his assessment of the insurgent threat in Iraq contradicted that of Mr. Rumsfeld or other officials; he said that the description of "guerrilla tactics" was proper "in strictly military terms."

But General Abizaid said that, at present, the force of about 147,000 American troops and 13,000 allied forces on the ground in Iraq was sufficient.

"I think our current force levels are about right," he said. "If the situation gets worse, I won't hesitate to ask for more."

General Abizaid also said that anti-American forces had fired two surface-to-air missiles at American aircraft within the last two weeks; one of them was today. Those attacks on C-130 cargo planes also indicate an escalation in the weapons used against allied forces, beyond automatic rifles and rocket-propelled grenades.

"Matter of fact, I was on the deck of a C-130 the other day, and we had a missile warning," General Abizaid said. "And the guy made a hard right bank. And we fired off all of our flares and, you know, we looked out there. And these were guys from the Oklahoma National Guard, and they actually thought it was fun. I was terrified."

Pentagon officials also disclosed today that there have been about five deaths among troops assigned to the Iraq mission that commanders say might have been suicides. As inquiries continue, one official said the suspected suicides were not clustered in any single time period that might indicate a related cause.