The United Nations says its chief representative in Iraq, Sergio Vieira de Mello, has died in the bomb-damaged wreckage of the U.N. headquarters in Baghdad.
A spokesman says the truck bomb that exploded this afternoon hit just under Mr. Vieira de Mello's office.
At least 15 others were killed, and many others were wounded in the attack.
The blast caused a large part of the U.N. compound to collapse. American military helicopters arrived on the scene and were helping in the rescue effort, flying the injured to hospitals. A column of black smoke could be seen coming from the blast area.
U.S. President George W. Bush called the bombing the work of "enemies of the Iraqi people" - terrorists who fear progress and hate peace.
The number of people in the building at the time is still unclear, but there are on average between 200 and 300 Iraqi and foreign staff that work in the compound.
The United Nations headquarters is in a building that was formerly the Canal Hotel and has been used by the U.N. for many years. United Nations weapons inspectors were based in the compound during their mandate in the 1990s.