WASHINGTON -- After yesterday's brutal attacks on American civilians in Iraq, President Bush and his aides insisted progress continues there and vowed not to back away as the United States did after grisly images of U.S. soldiers emerged from Somalia in 1993.

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Officials at the White House and State Department condemned as "horrific" the burning and mutilation of the remains of four U.S. government contractors, who were dragged through the streets of Fallujah by a celebrating mob.

White House press secretary Scott McClellan said the killings were "designed to intimidate and roll back the democratic progress and the freedom for the Iraqi people that we are achieving."

"The best way to honor those who have lost their lives is to continue to show resolve in the face of these cowardly, hateful acts," McClellan said.

Bush did not speak about the attacks during the day, and did not specifically mention them last night at a Washington fund-raiser for his campaign. He included his standard assertion that because he confronted Saddam Hussein, "an example of democracy is rising at the very heart of the Middle East ... the world is more free and ... America is more secure."

"We still face thugs and terrorists in Iraq who would rather go on killing the innocent than accept the advance of liberty," he said. "This collection of killers is trying to shake our will. America will never be intimidated by thugs and assassins."

Five U.S. Army soldiers were also killed yesterday when a roadside bomb exploded next to their armored vehicle convoy west of Baghdad, bringing the death toll for U.S. forces in March to at least 48. That is the second highest monthly total since Bush declared major combat operations over on May 1. The deadliest month was November, with 82 dead.

The twin assaults pose considerable challenges for Bush, who is trying to lay groundwork for ending the occupation on June 30 at the same time his handling of Iraq has become a major issue in the presidential campaign and on Capitol Hill. His former aide Richard Clarke charged last week that the invasion had been a diversion from more pressing fronts in the war on terrorism.

Retired Marine Gen. Anthony Zinni, a critic of the Iraq war and former head of the U.S. Central Command who was Bush's special envoy to the Middle East, said the violence was likely to "scare off international participation," making it more difficult for the White House to attract both foreign investment and military resources from other countries.

"We're going to find ourselves increasingly alone in this," said Zinni, who added that the Fallujah attacks were likely to lead to a crackdown by occupiers that will result in more anti-American images in the Arab media.

The graphic footage threatened to make the bloodshed more vivid for voters in the United States. The images of the bodies were instantly available on the Web, while more sanitized footage was shown on television. McClellan said the administration hopes that news organizations would act "responsibly in their coverage."

Edited footage of the attacks aired throughout the day on the cable news networks, and on the nightly news broadcasts. All of the networks had access to the same video, which was shot by a camera crew employed by the Associated Press.

The graphic nature of the images caused anchormen on ABC, NBC and CBS to warn viewers that the pictures were disturbing. CBS's Dan Rather, for example, introduced the "gruesome" report by emphasizing that it was "not for children's eyes."

The networks aired wide-angle shots of the attack, and pictures of crowds cheering and vehicles burning. But some, such as Fox News Channel, decided not to show pictures of the bodies being dragged, or burning or hung from a bridge tower. "We thought it might be too graphic," said Bill Shine, FNC's vice president of production. "We're on all day, and at any moment, we know kids can be watching." Another consideration, he said, was that Fox was not sure the victims' next of kin had been notified before the images hit the air.

NBC did show the dragging of the victims, but the footage was taken at a long angle, with the crowd obscuring most of the corpses, said "NBC Nightly News" executive producer Steve Capus.

"We accurately described what happened today while being sensitive to our viewers," he said. "I think that we conveyed the horror." McClellan was asked at a briefing if yesterday's deaths threatened to become a "Mogadishu moment," a reference to the horror in the United States at the images of a soldier's body being dragged through Somali streets following the slaughter of 18 commandos. President Clinton began withdrawing troops within weeks.

"Let me make it very clear that the Iraqi people are starting to realize freedom and democracy," McClellan replied. "We will continue to work with the Iraqi people and stay the course."

Administration officials maintain that vast areas of Iraq are safe and getting safer, but such violent images make the U.S. descriptions look out of step with reality.