The blast tore off a corner of the Canal Hotel housing UN offices
Rescuers are still searching the devastated Iraq headquarters of the United Nations in Baghdad, where a bomb blast killed at least 17 people including top UN envoy Sergio Vieira de Mello.
More than 100 people were injured in Tuesday's blast, which brought down three floors of the concrete building as a UN press conference on mine clearance was being held.
Many people are believed to be still trapped in the rubble.
The US military said the blast was caused by a bomb in a cement truck and was possibly a suicide attack. The truck was parked just outside de Mello's office when the device went off at about 1640 (1240GMT).
The 55-year-old Brazilian diplomat died after being trapped under rubble for several hours.
UN Secretary General Kofi Annan condemned the attack as a "crime not only against the United Nations but against Iraq itself".
"Nothing can excuse this act of unprovoked and murderous violence against men and women who went to Iraq... to help the Iraqi people," he said in a statement.
Mr Annan interrupted his vacation in Finland to return to New
York, where he will brief the UN Security Council on the bombing on Wednesday.
The dead also included the Iraq coordinator for the UN children's fund Unicef and several World Bank staff.
It is believed to be the most devastating attack on a UN
civilian complex in the body's 58-year-old history.
At UN headquarters in New York, all the national
flags ringing the entrance were taken down and the UN flag flew at half-mast.
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".
"The civilised world will not be intimidated," he said.
The UN compound was guarded by American troops.
The BBC's Susannah Price in Baghdad says the attack is a huge blow to the coalition's hopes of demonstrating they can bring the security and stability that ordinary Iraqis crave.
The blast came only hours after Mr Bush had 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 bombing follows a spate of attacks by shadowy fighters 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.