WASHINGTON, March 30 — The United States vehemently insisted today that there would be no pause or reassessment of the war in Iraq, despite complaints from officers in the field who say they are facing resistance of a sort they did not expect, and a suicide bombing on Saturday that indicated a significant change in tactics by the Iraqis.

"We have no plans for pauses, cease-fires or anything else," said Defense Secretary Donald H. Rumsfeld. And Gen. Richard B. Myers, chairman of the Joint Chiefs of Staff, added, "Nobody ever promised a short war."

In fact, several senior members of the Bush administration did expect a short war last year during the internal debate over whether to attack Iraq.

"Support for Saddam, including within his military organization, will collapse with the first whiff of gunpowder," Richard Perle, who resigned last week as chairman of the Pentagon's Defense Policy Board, predicted last summer. Vice President Dick Cheney and others made similar statements, some as recently as two weeks ago, helping to set up the problems the administration is facing now.

More than 125,000 British and American troops are now inside Iraq, the Pentagon said, with more crossing the border by air, land or sea every day. An additional 100,000 troops are on their way to Kuwait.

All this is according to plan, Mr. Rumsfeld and others said today. But over the weekend, Maj. Gen. Stanley A. McChrystal told a news conference that one Army unit, the 5,000 men of the Second Armored Cavalry Regiment, would likely be rushed to the battlefield faster than anticipated.

"There are discussions under way about potentially moving up part of its deployment," he said.

Despite the confident talk from Washington, many soldiers and even some other members of the coalition are saying they did not plan for what they are facing.

"I think they underestimated the role of the militias within the small cities and towns," said Robert Hill, defense minister for Australia, which has 2,000 troops in the Gulf. "They've certainly resulted in some delays in progress." He was speaking on Australian television today.

Brig. Gen. John F. Kelly, assistant commander of the First Marine Division in Baghdad, said, "Their determination is somewhat of a surprise to us."

Speaking on the CBS News program "Face the Nation," General Myers said he had full faith in the war plan, adding "I just can't explain why people are sniping at it." And Mr. Rumsfeld, speaking on the ABC News program "This Week," said, "We're within 49 miles of Baghdad, and there are all these people hyperventilating that this isn't working."

Some of the confusion may merely be semantics. The Pentagon had long planned to continue moving troops into Iraq even after the fighting had begun. But the hope, officials said, was that these forces would be used to stabilize the country after the regime of Saddam Hussein, the Iraqi president, was deposed. Now they will be needed to fight the war.

General Myers said the war plan contemplated two distinctly different realities, ranging from what he called "catastrophic success" to "a hard tough fight."

The New Yorker magazine, in its issue dated March 31, said Mr. Rumsfeld repeatedly rejected advice from senior military officers who said a far larger force was needed for an invasion of Iraq. At least six times in the lead-up to the conflict, the article said, Mr. Rumsfeld insisted that the number of troops be reduced, and each time he got his way.

Asked about that today, Mr. Rumsfeld said, "That's false, absolutely false." He explained that, early on, he told the generals to scrap the old plan "on the shelf," presumably from the first Gulf War. But the present plan, he added, was written by Gen. Tommy R. Franks, head of the Central Command, though "I wish I could take credit for it."

Both Mr. Rumsfeld and General Myers said they were not overly concerned by the suicide bombing on Saturday that took the lives of four American soldiers — and the pledge from the leadership in Baghdad that it was the first of many.

"It's one of those regrettable things," Mr. Rumsfeld said, "but they can't win here. Is it going to change the outcome? Not a chance."

General Myers added: "We are prepared to deal with that, but it's more harassment than militarily significant. Those are acts of desperation."

General Myers explained that some of the disagreement between forces on the ground and the military leadership results from their different perspectives.

"If you are in a firefight, for you that is the whole war," he said. "But when you get up to the operational or strategic level, you have a different view. Sometimes where you stand depends on where you sit."

He added that the forces in Iraq have yet to find any chemical or biological weapons, though troops have discovered hundreds of gas masks and protective suits. Still, he said "there is no doubt they have chemical weapons, and have them loaded in artillery shells."

In the weeks leading up to the war and its first few days, the Bush administration stressed that the primary motivation for the war was the need to find and destroy Iraq's weapons of mass destruction. But late last week the emphasis changed, perhaps because no such weapons have been found.

On Thursday and Friday, President Bush and Mr. Rumsfeld spent a lot of time talking about the brutality of the Iraqi regime. And at a briefing on Saturday, the Pentagon spokeswoman, Victoria Clarke, showed videos of Iraqis civilians killed in a chemical-weapons attack by the Hussein regime 15 years ago and a contemporary interview with an Iraqi woman who described torture and rape at the hands of the regime.

That sort of presentation was unusual for a news briefing, but Ms. Clarke said, "People seem to have been surprised at the brutality, at what the Iraqi regime is doing to some of their people."

Mr. Rumsfeld continued the theme today on the NBC News program "Meet the Press," when he talked about fedayeen forces that the Bush administration now calls "death squads."

"Already they have deployed death squads into Iraqi cities to terrorize civilians and to try to prevent them from welcoming coalition forces," he said, "to compel the regular army to fight by putting guns to their head, because they know that the only way to force them to fight is Saddam Hussein. The death squads report to the Hussein family directly."

General Myers said he had no idea whether Mr. Hussein was still alive or dead but added that what the administration called the "shock and awe" bombing campaign appeared to have worked, at least in part.

"If the best they can put forward" to speak for the country "is their information minister, with troops 50 miles outside of Baghdad, I would say they are in shock," General Myers said.