The United Nations will continue its work in Iraq despite Tuesday's devastating attack on its headquarters in Baghdad, Kofi Annan has said.
The blast was caused by a truck bomb
The UN's secretary general said it would not be distracted by what he called a senseless and brutal act - but expressed disappointment that the US-led military forces had failed to create a secure environment for the UN's work.
Mr Annan is currently meeting members of the UN Security Council in New York to review security arrangements for the UN's mission in Iraq.
As the search for clues continued at the scene of the attack, American investigators said they had found human remains inside the truck used in the blast, heightening speculation it was a suicide bombing.
At least 18 people were killed, including the UN special representative for Iraq, Sergio Vieira de Mello. More than 100 people were injured in the blast.
The US Federal Bureau of Investigation (FBI), which is helping Iraqi police investigate the bombing, said up to 1,500 lb (680 kg) of explosives, including shells and grenades, were used in the attack.
The FBI is trying to track down vehicle registration records in Iraq to find the possible number plate of the truck, which had been thrown over 450 metres (1,500 feet) in the blast.
A mechanical digger has been working through the huge mound of compacted concrete left by the explosion, after the search for survivors was scaled down.
Turkey is reportedly to send a rescue team to Iraq on Wednesday to help search for people missing in the rubble.
No group has yet claimed responsibility for the attack.
FBI officer Thomas Fuentes said it was speculation whether the culprits were foreigners or former fighters from Saddam Hussein's regime.
A member of Iraq's interim governing council, Ahmad Chalabi, said the council warned the United States of a possible terror attack days before the blast at the UN headquarters.
DYING IN THE UN'S SERVICE
1948: Mid-East mediator Folke Bernadotte of Sweden is killed by
1961: Secretary General Dag Hammarskjold dies in a plane crash on his way to negotiating a ceasefire in the former Belgian Congo
He said he received information on 14 August that a truck would be used in a large-scale terror attack "aimed at a soft target in Baghdad".
The European Commission is recalling some of its staff from Baghdad following the bombing.
UN spokesman Fred Eckhard said only two out of about 300 UN staff in Baghdad had accepted an offer of voluntary repatriation.
Heading into the Security Council meeting, Kofi Annan said the UN will continue with its work in Iraq and make a "judgement on additional requirements", such as improved security.
The explosion at the UN's headquarters is one of the most devastating attacks on a UN civilian complex in the body's 58-year history.
The truck used in the attack exploded just outside Vieira de Mello's office.
Many tributes have been paid to the 55-year-old Brazilian diplomat, who died after being trapped under rubble for several hours.
Mr Annan said he could think of "no-one we could less afford to spare, or who would be more acutely missed throughout the UN system" than Vieira de Mello.
He said those who died in the attack had been working to help the people of Iraq take control of their own destiny.
Among the dead are also the Iraq co-ordinator for the UN children's fund Unicef and a number of World Bank staff, as well as UN staff from the Philippines, Egypt, the UK, the US and Canada.
It also emerged that a Spanish naval captain, Manuel Martin Oar, working for Spain's special ambassador to Iraq, died of his injuries on Wednesday, bringing the death toll to at least 18 killed in the attack.
There are also a number of Iraqi casualties.
Meanwhile, violence against the coalition authorities continued.
In Saddam Hussein's hometown of Tikrit, fighters attacked a US convoy with a rocket-propelled grenade and small arms, killing a civilian and wounding two soldiers, the US military said.
UN HEADQUARTERS BOMBING
1. Truck seen on access road next to UN compound before it exploded.
2. Explosion destroyed three storeys, including office of Sergio Vieira de Mello
3. Explosion left crater nearly five metres across and two metres deep