FALLUJAH, Iraq - Insurgents attacked a U.S. military convoy and a Humvee was burned today near Fallujah, witnesses said, a day after the grisly killing and mutilation of four American contract workers in the city. The top U.S. administrator in Iraq said the deaths would not go unpunished.

It was not clear if there were any casualties in today's assault. Associated Press Television News footage showed smoke pouring from the vehicle that had been abandoned on a roadside just outside the city. Witnesses said the Humvee was hit by a rocket-propelled grenade.

U.S. officials said they could not confirm the attack.

Also today, two explosions near a U.S.-escorted fuel convoy in northern Baghdad wounded at least one Iraqi, witnesses said. APTN footage showed U.S. soldiers putting a wounded person on a stretcher inside an armored vehicle.

In Fallujah, police retrieved the remains of the four slain Americans on Wednesday night, wrapped them in blankets, and gave them to U.S. forces, said Iraqi police officer Lt. Salah Abdullah.

"We were shocked because our Islamic beliefs reject such behavior," he said, referring to the abuse of the bodies.

The top U.S. administrator in Iraq, L. Paul Bremer, today condemned the killings and said "their deaths would not go unpunished."

"Yesterday's events in Fallujah are dramatic examples of the ongoing struggle between human dignity and barbarism," he said at a graduation ceremony for police cadets. "The acts we have seen were despicable and inexcusable. ... They violate the tenets of all religions, including Islam, as one of the foundations of civilized society."

Iraqi police manned roadside checkpoints in and around Fallujah, but no U.S. troops could be seen inside. Shops and schools were open.

Some residents vowed to repel any U.S. forces.

"We will not let any foreigner enter Fallujah," said Sameer Sami, 40. "Yesterday's attack is proof of how much we hate the Americans."

Another resident, Ahmed al-Dulaimi, 30, said, "We wish that they would try to enter Fallujah so we'd let hell break lose."

Iraqi Interior Minister Nori al Badran vowed to send forces into Fallujah "to bring killers to justice," but did not say when that would happen.

"Forces will be sent to Fallujah ... from the army, the police and from the civil defense (force)," he said.

There was no sign of a military buildup near Fallujah by midafternoon today.

At a U.S. base about two miles east of the city, 1st Lt. Wade Zirkle said Wednesday's attack was carried out by a "few bandits and terrorists ... who are terrorizing the city."

"It is our job to go there and maintain security in the city and we are making sure that something like that will not happen again," he said, when asked whether U.S. forces would enter Fallujah.

Frenzied mobs dragged the burned, mutilated bodies of the four American contractors through the streets and strung two of them up from a bridge after rebels ambushed their vehicles.

It was similar to the scene more than a decade ago in Somalia, when a mob dragged corpses of U.S. soldiers through the streets of Mogadishu, eventually leading to the American withdrawal from the African nation. The images were broadcast worldwide and became the subject of the book and movie "Black Hawk Down."

U.S. officials denounced the Fallujah violence and vowed to stay the course in Iraq.

The White House blamed terrorists and remnants of Saddam Hussein's former regime for the "horrific attacks."

"It is offensive, it is despicable the way these individuals have been treated," White House press secretary Scott McClellan said.

Referring to the planned June 30 transfer of sovereignty to Iraqis, McClellan said "the best way to honor those that lost their lives" is to continue with efforts to bring democracy to Iraq.

State Department spokesman Adam Ereli said the contractors, all men, "were trying to make a difference and to help others." Officials did not identify the dead or the nature of their work because the next of kin had not yet been notified.

The four worked for Blackwater Security Consulting of Moyock, N.C., which provides training and guard services to customers around the world. The company, a subsidiary of Blackwater USA, referred calls to a spokesman in suburban Washington, who declined comment beyond a statement that said Blackwater was a government subcontractor providing security for the delivery of food in the Fallujah area.

Privately owned Blackwater USA's range of services include providing firearms and small-group training for Navy SEALs, police department SWAT teams and former special operations personnel.