It's still too early to pinpoint responsibility for the blast, reports CBS News Correspondent David Martin, but U.S. officials say there are three groups working to undo the American occupation and reconstruction of Iraq:
United Nations workers were told to stay at home Wednesday after a cement truck packed with explosives blew up outside the offices of the top U.N. envoy in Iraq, killing him and 19 other people. At least 100 people were injured in the unprecedented attack against the world body.
Annan said he was to meet with the Security Council later in the day to discuss security arrangements for U.N. workers in Iraq.
"We will persevere. We will continue. It is essential work," Annan said at a news conference in Stockholm, where he stopped briefly before heading to U.N. headquarters in New York. "We will not be intimidated."
Annan said the U.N. plans to reevaluate its security measures.
In other developments in Iraq:
After an all-night effort to find survivors, the rescue operation appeared to have turned into a grim search for the bodies of the many people unaccounted for at the heavily damaged U.N. headquarters. U.S. soldiers maintained a large presence in the area and American Army trucks were coming and going from the compound.
Heavy machinery was pulling up the smashed pieces of the building, strewn akimbo by the blast.
Tuesday's bomb blasted a 6-foot-deep crater in the ground, shredding the facade of the Canal Hotel housing U.N. offices and stunning an organization that had been welcomed by many Iraqis in contrast to the U.S.-led occupation forces.
Except for a new concrete wall built recently, U.N. officials at the headquarters refused heavy security because the U.N. "did not want a large American presence outside," said Salim Lone, the U.N. spokesman in the Iraqi capital.
Still, it appears that the wall kept the bomber from entering the U.N. compound, but the vulnerable building next door, a hospital, was exploited, reports CBS News Correspondent Randall Pinskton. Someone managed to maneuver the truck bomb into a driveway, next to the U.N. headquarters.
"There are so many people who are still missing," said Veronique Taveau, a spokesperson for the U.N. Humanitarian Coordinator.
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. Taveau said Wednesday the U.N. figure for the dead was 17 and 100 people were wounded.
Taveau said the U.N. had temporarily suspended operations on Wednesday and that travel arrangements were being made for employees wanting to leave the country.
Iraqis who work for the U.N. were told to stay at home. Foreign workers were directed to stay in the lodgings that are scattered in many small hotels around the capital.
"Moving outside is forbidden," said Salam Quzaz, from the United Nations Development Program.
The 4:30 p.m. blast Tuesday severely damaged the office of the top U.N. envoy in Iraq, Sergio Vieira de Mello, who was meeting with other U.N. officials.
Vieira de Mello — a 55-year-old veteran diplomat serving in what one U.N. spokesman called the world body's toughest assignment — was wounded and trapped in the rubble, and workers gave him water as they tried to extricate him. Hours later, the United Nations announced his death.
U.N. and U.S. officials called the bombing a "terrorist attack," but there was no immediate claim of responsibility. The bombing came nearly two weeks after a car exploded and killed 19 people at the Jordanian Embassy in Baghdad and after a string of dramatic attacks on oil and water pipelines in Iraq.
Like the remote-controlled explosion at the Jordan Embassy, the suicide bombing on the U.N. headquarters focused on a high-profile target with many civilians inside and resembled attacks blamed on Islamic militants elsewhere in the world. It was far more sophisticated than the guerrilla attacks that have plagued U.S. forces, featuring hit-and-run shootings carried out by small bands or remote-control roadside bombs.
As FBI agents joined the investigation, Bernard Kerik, the former New York City police commissioner who is rebuilding the Iraqi police force, told reporters that evidence suggested the attack was a suicide bombing.
But he said it was "much too early" to say if Osama bin Laden's al Qaeda network was behind the attack. "We don't have that kind of evidence yet."