A huge bomb has devastated the Iraq headquarters of the United Nations in Baghdad killing at least 17 people, including top UN envoy Sergio Vieira de Mello.
Rescuers are frantically working to pull survivors from the rubble
The blast brought down three floors of the concrete building injuring more than 100 people.
Rescue efforts to find many more people still thought to be trapped in the rubble work are continuing into the night.
A US military spokeswoman said the blast was caused by a truck bomb - possibly in a suicide attack.
The attack came shortly after the UN increased security measures around its Baghdad building.
The truck containing the bomb was parked just outside de Mello's office when the device went off at about 1640 local time (1240GMT).
The 55-year-old Brazilian diplomat died after being trapped under rubble for several hours.
Even before his death was reported, UN Secretary General Kofi Annan cut short a holiday in Finland and decided to return to New York.
Mr Annan said in a statement that everyone at the UN was "shocked and dismayed" at the attack.
"Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq... to help the Iraqi people," the statement said.
The top US official in Iraq, Paul Bremer, vowed to pursue those behind the bombing.
"This is an awful time. We will leave no stone unturned to find the people who did this," he said.
UN HQ BLAST
Blast hit entrance of UN HQ
Offices destroyed on three levels
Explosion very close to UN special representative's 2nd-floor office
Building previously used by UN weapons inspectors and other UN agencies
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.
US President George W Bush, speaking from his ranch in Texas, blamed the attack on "terrorists" who he called "the enemies of the Iraqi people".
He said the attack on the UN building would not intimidate the civilised world from assisting the Iraqi people to reclaim their future.
The blast came only hours after Mr Bush welcomed the capture of a former Iraqi vice-president known as "Saddam's knuckles".
Taha Yassin Ramadan was seized in the northern city of Mosul on Monday and has now been handed over to the US Army.
The BBC's Middle East analyst Roger Hardy, says the attackers may have targeted the UN building because they considered the world body to be America's junior partner in the occupation of Iraq.
But most Iraqis do not agree, and seem to want a bigger a role for the UN, he says.
They tend to focus their hostility and resentment on the Americans, he adds.
The BBC's Valerie Jones in Baghdad says the blast was heard over a wide area of the Iraqi capital.
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.
The latest bombing in Baghdad follows a steady stream of attacks by groups opposed to the occupation of Iraq.
At least 60 US soldiers have died since 1 May, when President Bush declared an end to major combat operations in Iraq.
Utilities and other oil facilities have also been attacked.
US army engineers are currently struggling to contain a fire on Iraq's main northern oil export pipeline, following two attacks at the weekend.
On 7 August 11 people died in a bomb attack against the Jordanian embassy in Baghdad.
On taking up his new post last month, the new US commander for the Middle East and Central Asia, General John Abizaid, said coalition forces were facing what appeared to be a systematic guerrilla war.