Blast Rips U.N. Building in Baghdad

— The top U.N. official in Iraq was killed today after a massive truck bomb ripped through the world body's Baghdad headquarters. At least 16 others died and more than 100 were wounded in the most devastating attack ever on a U.N. facility.

The United Nations confirmed Sergio Vieira de Mello, the Brazilian who headed the U.N. mission in Iraq, had died. Vieira de Mello, 55, was in his office when the explosion occurred and was trapped in the rubble for hours.

U.N. officials said 17 people were killed, including seven U.N. employees. Of the 108 wounded, 86 have been hospitalized and 22 were hurt but walked away from the scene, U.N. officials said.

In a strong condemnation of the bombing, President Bush today called the attackers "enemies of the civilized world."

"These killers will not determine the future of Iraq," Bush told reporters at his ranch in Crawford, Texas. "Iraq is on an irreversible course toward self-government and peace. And America and our friends in the United Nations will stand with the Iraqi people."

In addition to Vieira de Mello, the U.N. employees killed in the blast were identified as Rick Hooper, an American who worked in the Department of Public Affairs; Fiona Watson, from the United Kingdom; Ranillo Buenaventura and Marilyn Manuel, both from the Philippines; Jean-Selim Kanaan, an Egyptian; and Chris Klein-Beckman, a 32-year-old Canadian who worked for the United Nations Children's Fund.

U.S. military officials in Baghdad said the bomb blew up near al Kanal hotel housing the U.N. headquarters in northeastern Baghdad around 4:30 p.m. local time. The hotel had been the headquarters for U.N. operations in Iraq since the 1991 Persian Gulf War, and housed the U.N. weapons inspection program and humanitarian assistance efforts.

A truck apparently approached al Kanal hotel from a parking area on the side of the building. A 12-foot-tall concrete wall had just been completed around the 600-by-900-foot hotel compound, but the site was not strictly guarded by police or military forces. It was unclear whether the truck tried to ram through the wall.

Annan Heading Back to New York

The light security presence was intentional, U.N. officials said, because they "did not want a large American presence outside," U.N. spokesman Salim Lone said.

"We are unarmed. We don't have a lot of security, as this bomb shows," Lone said. "We don't want a lot of security, because we're here to help the people of Iraq."

Messages of condolence and outrage came pouring in from officials and human rights groups around the world.

Shortly after the attack, a U.N. spokesman said U.N. Secretary-General Kofi Annan had cut short his vacation in northern Europe to return to New York, where the international body is headquartered.

In a statement released in New York, Annan expressed shock and dismay over the attack and said he hoped the perpetrators would be brought to justice.

The flags of the 191 U.N. member states were taken down, and the blue and white U.N. flag was lowered to half-staff.

Trapped in the Rubble

As night fell on Baghdad, distraught relatives of local U.N. staffers were desperately trying to reach the devastated site of the attack, anxious for news about their loved ones.

A security cordon was established around the area. Witnesses said Black Hawk helicopters hovered around the site ready to ferry the wounded to hospitals.

Reporting from Baghdad today, ABCNEWS' Mike Von Fremd said two people appeared to be buried alive in the rubble as rescuers were calling into a hole in the rubble.

Footage of scenes after the attack taken by a Japanese news crew showed wounded and panicked U.N. employees covered in dust and rubble scrambling to get out of the building.

Rescuers carried out the wounded and the dead by the dozens.One wounded man had a yard-long, inch-thick aluminum rod driven into his face just below his right eye, according to The Associated Press. He identified himself as a security consultant for the International Monetary Fund.

No Claim of Responsibility

There was no claim of responsibility for the explosion. But reporting from the Pentagon today, ABCNEWS' Martha Raddatz said U.S. military officials have been concerned about attacks on "softer" targets after weeks of guerrilla-style attacks on U.S. soldiers in Iraq.

According to ABCNEWS military consultant Tony Cordesman, today's bombing was part of a "systematic pattern of attacks mixed with sabotage" designed to convince Iraqis and people outside Iraq that the United States was failing in its mission in Iraq.

Earlier this month, 11 people were killed in a car bomb explosion at the Jordanian Embassy in Baghdad.

Cordesman warned that it would be difficult to figure out who was responsible for the attack.

The suspect list was long and wide, he suggested, and could include former servicemen in ousted dictator Saddam Hussein's security apparatus or militants from abroad or simply young Iraqis being paid to mount attacks.

The explosion occurred shortly after U.S. officials confirmed that Taha Yassin Ramadan, a former Iraqi vice president and No. 12 on the U.S. list of 55 most-wanted Iraqis, was captured and handed over to U.S. forces in the northern Iraqi city of Mosul.

Moments after the news of the capture, Bush hailed Ramadan's capture. "I like to remind people that a free Iraq will no longer serve as a haven for terrorists, or a place where terrorists can get money or arms," he told reporters in Crawford.

ABCNEWS' John Berman, Mike Von Fremd and Tina Babarovic in Baghdad and Martha Raddatz in Washington contributed to this report.