A deadly explosion that tore through the United Nations headquarters in Iraq raised concerns Islamic militant groups were infiltrating the postwar country while unifying world leaders in their resolve against terrorism.

Nations that had previously been divided over the war in Iraq spoke in a chorus of condemnation after Tuesday's bombing.

"Acts as odious as this can only prompt indignation and unreserved condemnation," French President Jacques Chirac (search) said in a message to U.N. Secretary-General Kofi Annan.

Meanwhile, a U.N. official told Reuters news service there were no plans for a mass evacuation of staff in Iraq. Earlier reports indicated an evacuation of all Iraq staff to Jordan would be ordered.

A U.N. spokesman said Tuesday's explosion, if confirmed to be the work of a homicide bomber, would be the first attack of its kind on a U.N. facility. Occupying U.S. forces have come under attack almost daily from suspected Saddam loyalists since President Bush declared the end of major combat May 1.

But the target and style of the bombing raised concerns that Saddam loyalists were no longer restricting attacks to GIs -- dozens of Iraqis worked in the U.N. mission -- and that outside Islamic militant groups had moved in to take advantage of the postwar nation's fragile state.

• Map: Recent Developments in Iraq
• Video: Inside the U.N. Mission
• Video: Brit Hume Talks to Paul Bremer

The blast, which the U.N. Security Council (search) called a "terrorist attack," killed 20 people and injured at least 100 others. Workers dug through the rubble of Baghdad's Canal Hotel, which housed the U.N. offices, searching for survivors.

Reuters news service reported Wednesday that a U.N. spokesman expressed fears that there ar emore bodies trapped under the rubble.

At around 4:30 p.m. local time, a large cement truck packed with explosives detonated at the concrete wall outside the three-story Canal Hotel while a news conference was under way in the building, where 300 U.N. employees work.

"The explosion was caused by a massive truck bomb," said Bernard Kerik (search), the senior U.S. law enforcement official in Baghdad. "We have evidence to suggest it could have been a suicide attack."

The FBI will be leading the investigation into the blast, Fox News has learned.

President Bush also labeled the blast an act of terror: "By attempting to spread chaos and fear, terrorists are testing our will. Across the world, they are finding that our will cannot be shaken."

Kerik also cautioned it was "much too early" to say if Al Qaeda was behind the attack.

The attack was eerily similar to the Aug. 7 car bomb attack on the Jordanian Embassy in Baghdad (search), which killed 11 people and left more than 50 wounded. Dia'a Rashwan, an expert on radical Islam at Egypt's Al-Ahram Center for Political and Strategic Studies, said the attack fits "the ideology of Al Qaeda. They consider the U.N. one of the international actors who helped the Americans to occupy Palestine and, later, Iraq."

The head administrator of postwar Iraq, L. Paul Bremer (search), said the blast may have specifically targeted Sergio Vieira de Mello (search), a top U.N. official who was among the dead. "The truck was parked in such a place here in front of the building that it had to affect his office," Bremer said.

Vieira de Mello, a 55-year-old veteran diplomat serving in what one U.N. spokesman called the world body's toughest assignment, was meeting with other U.N. officials in his office when the explosion brought the room down around them.

"It's a tragic personal loss but it's also, on a professional and political level, a big loss for the U.N.'s efforts in Iraq, " U.N. spokesman Fred Eckhard told Fox News. "Sergio was one of the big achievers … that's why he was chosen for this job, which is one of the toughest, if not the toughest" job in that country.

Vieira de Mello began work June 2 and would have finished his assignment at the end of September, though U.N. spokesman Salim Lone said many U.N. officials wanted him to stay on.

At least one American was also killed in the blast: Richard Hooper, 40, of Walnut Creek, Calif., the special assistant to the U.N. Undersecretary-General for Political Affairs. A State Department consular officer said the U.S. military was working to identify any other Americans who may have been injured in the explosion or who were in its vicinity.

The attack, which killed and wounded U.N. workers from around the globe, was one of the worst in the organization's history, one official said. U.N. spokesman Eckhard said the probable homicide bombing "would, to my knowledge, be the first on a U.N. facility." It was the worst attack on a U.N. facility since Israeli forces, responding to a Hezbollah attack, bombarded a U.N. compound at Qana in southern Lebanon in April 1996, killing 91 refugees.

The blast also prompted questions about the nature of the U.N. mission in Iraq, a country that had previously welcomed the organization's presence despite its ties to the United States.

Except for a newly built concrete wall, U.N. officials at the headquarters refused the sort of heavy security that the U.S. military has put up around some sensitive civilian sites. The United Nations "did not want a large American presence outside," the U.N.'s Lone said.

The blast, the shock at being targeted and the death of a rising star beloved in the organization struck deep. All the national flags that ring the U.N. headquarters' entrance in New York were removed from their poles, and the blue-and-white U.N. flag was lowered to half-staff. Staffers, tears in their eyes, gathered in hallways and watched in shock as televisions reported on his death.

"Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq for one purpose only: to help the Iraqi people recover their independence and sovereignty, and to rebuild their country as fast as possible, under leaders of their own choosing," Secretary-General Annan said in a statement.

Nonetheless, U.N. officials vowed to continue their mission in Iraq.

Deputy Syrian ambassador Fayssal Mekdad, whose country holds the Security Council presidency, said "such terrorist incidents cannot break the will of the international community" and that U.N. programs would continue.

The United Nations distributes humanitarian aid and is developing programs aimed at boosting Iraq's emerging free press, justice system and monitoring of human rights. United Nations weapons inspectors worked out of the hotel during the period before the war.

Fifteen bodies in white bags were counted by a U.N. worker at the hotel, and a survey of Baghdad hospitals by The Associated Press found five other people who had died in the blast. U.N. officials said 14 of those killed were U.N. workers and 100 people were wounded.

An AP reporter counted 40 wounded people lying in the front garden and receiving first aid. Some were loaded into a constant stream of helicopters that ferried the injured away. A senior UNICEF official also was seriously wounded in the blast, U.N. officials said.

"I can't move. I can't feel my legs and arms. Dozens of people I know are still under the ruins," Majid Al-Hamaidi, 43, a driver for the World Bank, cried out.

Bremer, the top U.S. administrator, walked through the scene of destruction as workers dug through the rubble with their hands. Dozens of U.S. Humvees were at the scene and at least two Black Hawk helicopters hovered above.

Bremer had tears in his eyes and hugged Hassan al-Salame, an adviser to Vieira de Mello. A part of the building collapsed near him. People cried: "Watch out. Watch out."

"We will leave no stone unturned to find the perpetrators of this attack," he said.

One wounded man had a yard-long, inch-thick aluminum rod driven into his face just below his right eye. He identified himself as a security consultant for the International Monetary Fund, saying he had just arrived in the country over the weekend.

Tuesday's attack, which used twice as many explosives as in the Jordanian Embassy bombing, was far more sophisticated than the guerrilla attacks that have plagued U.S. forces, which have mostly featured hit-and-run shootings carried out by small bands or remote-control roadside bombs.

Nations that had criticized the war in Iraq announced their support for postwar peace in the nation.

Arab League Secretary-General Amr Moussa, a sharp critic of the U.S. role in Iraq, said his organization "expresses its solidarity with the United Nations and demands that the international community support the U.N. mission in Baghdad and ensure its protection."

And Chancellor Gerhard Schroeder of Germany, who strongly opposed military action in Iraq, said, "This is a criminal attack, clearly carried out by forces that do not want the rebuilding of Iraq to take place in peace and freedom."

Fox News' Jane Roh and The Associated Press contributed to this report.