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Sergio Vieira de Mello (search), a diplomatic veteran who served for more than 30 years as a troubleshooter in world hotspots and became the top U.N. envoy in Iraq, was killed Tuesday in a truck bombing against his offices in Baghdad. He was 55.

A rising star within the world body, the Brazilian envoy's skills were so admired that he had been talked about as a future secretary-general.

After being named to the Iraq post in June, the diplomat said his top priority was to protect the interests of the Iraqi people under the U.S.-led occupation.

"I have been sent here with a mandate to assist the Iraqi people and those responsible for the administration of this land to achieve ... freedom, the possibility of managing their own destiny and determining their own future," he said on arrival in Baghdad.

Vieira de Mello had reluctantly agreed to leave his job as U.N. High Commissioner for Human Rights and accept a four-month assignment as the top envoy in Baghdad at Secretary-General Kofi Annan (search)'s request. He had been the top choice of the United States.

In a briefing to the U.N. Security Council just 3 1/2 weeks ago, he warned that security in Iraq remained tenous and declared that "the United Nations presence in Iraq remains vulnerable to any who would seek to target our organization."

"Our security continues to rely significantly on the reputation of the United Nations, our ability to demonstrate, meaningfully, that we are in Iraq to assist its people, and our independence," he said.

All the national flags that ring the U.N. headquarters' entrance in New York were removed from their poles. The blue and white U.N. flag was lowered to half staff.

Annan said Vieira de Mello's death was "a bitter blow for the United Nations, and for me personally."

"Those who killed him have committed a crime, not only against the United Nations but against Iraq itself," Annan said.

The U.N. chief called the man whose career he nurtured "an outstanding servant of humanity." He urged that the work he began "be completed so that his death will not have been in vain."

U.N. spokesman Fred Eckhard said the diplomat "was a terribly focused, energetic, determined and talented man."

President Bush said Vieira de Mello committed his life to advancing the cause of human rights and was selflessly "helping the Iraqi people move down the path towards a democratic country governed by the rule of law."

Human rights organizations also mourned his death.

"It is tragic he should end up the victim of the kind of war crime he fought so hard to prevent," said Kenneth Roth, executive director of Human Rights Watch.

Vieira de Mello's Iraq posting capped a career as the United Nations' crisis point man, sent to conflict zones to help end the bloodshed and rebuild in the aftermath.

When Annan introduced Vieira de Mello in late May as his new special representative for Iraq, he said his "exceptional and unique experience" in running post-conflict U.N. operations in Kosovo (search) and East Timor (search), and his reputation "as a good team builder and a consensus builder" made him the perfect choice.

In Iraq, he had to rely on both diplomacy and tough talk as the United Nations tried to find its place after the Iraqi war come close to rendering it obsolete. He had to work with the United States, even while distinguishing the world body from the occupation, unpopular with many Iraqis.

In his last interview published before his death, Vieira de Mello sympathized with Iraqi resentment at having foreign troops on their soil.

"It is traumatic. It must be one of the most humiliating periods in their history. Who would like to see their country occupied? I would not like to see foreign tanks in Copacabana," Vieira de Mello told the Brazilian newspaper Estado de Sao Paulo in an interview published Monday.

Throughout his tenure in Baghdad, he took pains to remind everybody that the United Nations would be in the country long after U.S. forces leave and insisted that the world body -- not the U.S.-led coalition -- should control the spending of Iraqi oil revenues.

"We're truly in a unique situation here," he said of the occupation. "By the usual U.N. standards, this is at best a bizarre situation."

Born in Rio de Janeiro on March 15, 1948, Vieira de Mello studied philosophy at the prestigious Sorbonne University in Paris before embarking on a career in the United Nations, rapidly gaining a reputation as a smooth-talking, hands-on operator.

He joined the U.N. High Commissioner for Refugees in 1969, serving in Bangladesh during its independence in 1971. Following the 1974 Turkish invasion of Cyprus he worked with refugees on the island.

He spent three years in charge of UNHCR operations in Mozambique during the civil war that followed its independence from Portugal in 1975, and three more in military-ruled Peru. He became senior political adviser to the U.N. Interim Force in Lebanon between 1981 and 1983, covering the period when Israel invaded.

Vieira de Mello then returned to the UNHCR, working at its head office in Geneva for 10 years.

The early 1990s found him in Cambodia and then in disintegrating Yugoslavia. After working on the refugee problem in central Africa,he was made assistant high commissioner for refugees in 1996 and he became undersecretary-general in New York two years later.

He was a special U.N. envoy in Kosovo following the U.S.-led bombing raids that broke Serbian control of the Yugoslav province in 1999.

When Indonesia withdrew from East Timor later that year, Vieira de Mello was sent there, gaining widespread praise for overseeing the country's three-year transition to independence.

"You don't change the devastation of 1999 into a Garden of Eden in 2 years," he said. But he said U.N. workers had "laid solid bases for the country to live in peace."

Vieira de Mello was divorced and had two sons. His mother lives in Rio de Janeiro.