WASHINGTON - On his first day as defense secretary, Robert Gates warned Monday that failure in Iraq would be a "calamity" that would haunt the United States for years. Underscoring eroding security there, a Pentagon report said the number of insurgent and sectarian attacks had risen to the highest level in years.

Sworn into office as the Bush administration moves toward revamping its strategy in Iraq, Gates sketched out an agenda of reversing the downward spiral in Iraq, attending to resurgent violence in Afghanistan and pushing for the military modernization that was a priority of his predecessor, Donald H. Rumsfeld.

Gates said he intends to travel soon to Iraq to hear commanders' assessments of the situation on the ground and to gain their advice — "unvarnished and straight from the shoulder" — on how to adjust U.S. war strategy. He said he would give President Bush honest advice and listen to military commanders — a contrast to critics' complaints that Rumsfeld was an ideologue who paid scant heed to top officers.

"All of us want to find a way to bring America's sons and daughters home again," Gates told a few hundred people in a Pentagon auditorium, including Bush, Cabinet secretaries, members of Congress, the Joint Chiefs of Staff and Gates' wife and mother. Rumsfeld, who handed off his authority earlier Monday in a private event, did not attend the ceremony.

"As the president has made clear," Gates said, "we simply cannot afford to fail in the Middle East. Failure in Iraq at this juncture would be a calamity that would haunt our nation, impair our credibility and endanger Americans for decades to come."

Gates has not tipped his hand on the kinds of changes in Iraq strategy he thinks may be needed. He said that since his Senate confirmation in early December he has held in-depth discussions with Bush on Iraq policy.

More broadly, Gates has said he will keep an open mind about other issues at the Pentagon, including proposals by the heads of the Army and Marine Corps to increase the size of their services to cope with the strains of war. Last week, Gen. Peter Schoomaker, the Army's top commander, warned that his force "will break" without thousands more active duty troops and greater use of the reserves.

Retired Army Brig. Gen. Kevin Ryan, a senior fellow at the Belfer Center for Science and International Affairs at Harvard University, said in an interview Monday that he feels certain Gates will have the latitude within the administration to push for a bigger Army and Marine Corps. "The question is going to be how high" to go, Ryan said.

At the Pentagon ceremony, Bush said he is confident Gates, 63, will bring a fresh perspective to the Iraq problem.

"He knows the stakes in the war on terror," Bush said. "He recognizes this is a long struggle against an enemy unlike any our nation has fought before. He understands that defeating the terrorists and the radicals and the extremists in Iraq and the Middle East is essential to leading toward peace."

Bush made no mention of his plan for changing Iraq strategy, which he has said will be unveiled next month.

Amid growing speculation that Bush will choose to send tens of thousands more U.S. troops to Baghdad in a reinvigorated attempt to quell the sectarian violence, a leading Democrat in Congress cautioned against that move.

"Everything I've heard and everything I know to be true lead me to believe that this increase at best won't change a thing, and at worst could exacerbate the situation even further," said Rep. Ike Skelton (news, bio, voting record), the Missouri Democrat who will become chairman of the House Armed Services Committee in January.

Gen. James Conway, the new commandant of the Marine Corps, told reporters in Missouri on Saturday that one option Bush is considering is to add five or more combat brigades — roughly 20,000 troops. He said he believes the Joint Chiefs as a whole would support adding that number in Iraq "if there is a solid military reason for doing so." He stressed that adding troops just to be "thickening the mix" in Baghdad would be a mistake.

U.S. commanders moved several thousand more U.S. troops into Baghdad last summer in a bid to tamp down the violence. The move worked briefly, but the violence rebounded quickly, according to the Pentagon report sent to Congress on Monday.

The report said attacks on U.S. and Iraqi troops and Iraqi civilians jumped sharply in recent months to the highest level since Iraq regained its sovereignty in June 2004. From mid-August to mid-November, the weekly average number of attacks increased 22 percent from the previous three months. The worst violence was in Baghdad and in the western province of Anbar, long the focus of activity by Sunni insurgents, the report said.

A bar chart in the Pentagon's report to Congress gave no exact numbers but indicated the weekly average had approached 1,000 in the latest period, compared with about 800 per week from the May-to-August period. Statistics provided separately by the Pentagon said weekly attacks had averaged 959 in the latest period.

The report also said the Iraqi government's failure to end sectarian violence has eroded ordinary Iraqis' confidence in their future. That conclusion reflects some of the Bush administration's doubt about the ability of Prime Minister Nouri al-Maliki to make the hard decisions U.S. officials insist are needed to quell the violence.

"The failure of the government to implement concrete actions in these areas has contributed to a situation in which, as of October 2006, there were more Iraqis who expressed a lack of confidence in their government's ability to improve the situation than there were in July 2006," the report said, calling for urgent action in Baghdad.

It made no mention of a timetable for ending U.S. military involvement.

It said that as security conditions permit and the Iraqi army and police become more capable, U.S. forces will move out of the cities, reduce the number of bases from which they operate and conduct fewer visible patrols. That has been the basic strategy for some time, but it has not been fully implemented because of the explosion in sectarian killings this year and disappointments in the pace of developing Iraqi security forces.

The development of an Iraqi army and police is making progress, the report said, but much remains to be done.

It said, for example, that the goal of training and equipping an Iraqi army of about 137,000 soldiers is 98 percent completed, although it also noted that far fewer troops are actually available for duty on any given day due to absenteeism, casualties, desertions and leaves of absence.

Lt. Gen. John Sattler, the plans and strategy chief for the Joint Chiefs, told reporters Monday that of the approximately 322,000 Iraqi troops and police now trained and equipped, only about 280,000 are available for duty.

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