An explosion has ripped through the United Nations headquarters in Baghdad, killing at least 13 people and injuring the top UN envoy to Iraq.
The blast tore away part of the building
Scores of people are reported to have been injured in the blast, which brought down three floors of the concrete building.
UN special representative Sergio Vieira de Mello was injured in the explosion and remains trapped in the rubble along with other wounded and dead.
A US military spokeswoman said the blast was caused by a truck or car bomb - possibly in a suicide attack.
The attack came shortly after the UN increased security measures around its Baghdad building.
It took place at about 1640 local time (1240GMT).
A UN spokesman in Baghdad, Salim Lone, called the explosion "an unspeakable crime against people from all over the world who have come here to help the people of Iraq".
In New York, UN secretary general Kofi Annan's spokesman Fred Eckhard described the attack as "a tragedy, I think, not only personally but also a setback politically for the UN mission".
But the Syrian diplomat who currently chairs the UN Security Council said the world body would not be dissuaded from its work.
"Such terrorist incidents cannot break the will of the
international community to further intensify its efforts to help the people of Iraq," Fayssal Mekdad said on behalf of the Security Council.
The BBC's Valerie Jones in Baghdad says the blast was heard over a wide area of the Iraqi capital.
US military forces are searching for survivors.
Mr Lone told the BBC rescue workers were struggling to free Mr de Mello from the wreckage of his office.
The blast took place "right below" his window, he said.
"I guess it was targeted for that.
"His office and the offices around him no longer exist - it is all rubble," he said.
He described flying glass and wood surrounding him as the blast hit his office.
A plume of black smoke was seen pouring out of the building, and injured people were seen being carried away by stretcher.
Some were taken to waiting medical helicopters.
Hundreds of people worked in the building
Hundreds of people work in the building, a converted hotel.
It would have been especially busy at the time of the blast because a press conference was taking place.
Our correspondent says the blast was even larger than one at the Jordanian embassy in Baghdad, which killed 11 people on 7 August.