Annan said the UN 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. 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.

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. Those who died in the attack, Annan said, had been working to help the people of Iraq take control of their own destiny.

Annan was speaking in Stockholm on his way back to New York after interrupting his holiday. He is to met members of the Security Council yesterday to evaluate the future of the UN mission in Iraq.

?We are reassessing our security arrangements in Iraq. We have been in Iraq for 12 years and we have never been attacked,? he said.

Turkey send a rescue team to Iraq yesterday to help search for people missing after the blast, said US Senator, Richard Lugar, who is on a visit in Ankara. 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.

Worst attack for UN

The explosion is believed to be the most devastating attack on a UN civilian complex in the body?s 58-year history. The US military said it 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. Many tributes have been paid to the 55-year-old Brazilian diplomat, who died after being trapped under rubble for several hours.

Acting UN High Commissioner for Human Rights, Bertrand Ramcharan, told reporters in Geneva : ?This was an immensely gifted man with intelligence, charm, and a solid commitment to the principles of the United Nations and the ideals of the Charter.?

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, had died of his injuries, bringing the death toll to at least 18 killed in the attack. There are also a number of Iraqi casualties.

Injured staff have been evacuated to Jordan for further medical treatment. Lone said the UN had chosen not to have intense security because they had come to help the people of Iraq. Flags have been flying at half-mast at UN offices.

The top US official in Iraq, Paul Bremer, vowed to pursue those behind the bombing. ?This is an awful crime. I can tell you we will leave no stone unturned, working with the Iraqi police to find the people who did this,? he said.