In New York, the U.N. flag is flying at half staff, as the world mourns the loss of United Nation's High Commissioner for Human Rights, Sergio Vieira de Mello, who was killed Tuesday in a car-bombing at the United Nations compound in Baghdad while serving there as the U.N.'s special envoy for Iraq.

Sergio Vieira de Mello's death has come as a devastating shock to the international community.

Over a 33-year career with the United Nations, the respected Brazilian diplomat and humanitarian made an enormous impact on some of the world's most devastated nations. He was an instrumental figure in the United Nations, leading U.N. missions to rebuild Kosovo, East Timor, and most recently, Iraq.

Upon hearing of Mr. de Mello's death, U.N. spokesman Fred Eckhard read a statement from Secretary-General Kofi Annan.

"The loss of Sergio Vieira de Mello is a bitter blow and for me personally," he said. "The death of any colleague is hard to bear. But I can think of no one we could less afford to spare, or who would be more acutely missed throughout the U.N. system than Sergio. Throughout his career, he has been an outstanding servant of humanity, dedicated to relieving the suffering of his fellow men and women, helping them to [resolve] their conflicts and rebuild their war-torn societies."

Mr. de Mello was midway through a four-month assignment as Mr. Annan's special representative to Iraq.

Mr. de Mello, a Brazilian national, was 55 years old. He is remembered as one of the world's top troubleshooters.

Mr. de Mello led U.N. human rights missions in Mozambique, and Peru, and was appointed principal adviser to U.N. forces in Lebanon in 1981. Later, following the genocide crisis in Rwanda, Mr. de Mello served as U.N. Humanitarian Coordinator there.

He became the United Nations' special representative for Kosovo in 1999. The next year he led U.N. operations in East Timor.

Last year, he became the head of the U.N. Human Rights Commission based in Geneva, a post he is said to have cherished.

When Mr. de Mello began his work in Iraq, he made it a priority to listen to Iraqis, and to work with Iraq's new potential leaders. Speaking at the United Nations at the end of May, Mr. de Mello expressed his enthusiasm for helping Iraq rebuild.

"Priority number one will be to establish contacts with representative Iraqi leaders, representatives of the media, of civil society, and there are many," he said. "Iraqi society is rich and that richness has been suppressed brutally for the last 24 years but they are there. They are there or they are returning as we speak and that is my priority."

Ambassador Adolfo Aguilar Zinser of Mexico stresses that Mr. de Mello's passing will not bring an end to the work that he was doing in Iraq.

"We know that the spirit, the vision, the optimism the courage, the energy of Sergio will remain in Iraq with the United Nations," he said.

Those who knew Mr. de Mello say he had a leader's charisma combined with a compassion for people that made him truly effective.

Ambassador William Luers, the president of the United Nations Association of the United States of America, a private group, maintained a close relationship with Mr. de Mello for many years.

"It's that skill of knowing people, figuring them out and getting to them, which is what a lot of the basic work of peace building is all about, and he's the master at it," he said.

Ambassador Luers, along with many longtime U.N. observers, believes that Mr. de Mello would have one day become secretary-general of the United Nations.