The United Nations and the United States made mistakes in Iraq, but the ultimate responsibility for a secure environment in that country lies with the US-led coalition forces, Secretary-General Kofi Annan said on Wednesday.

Annan, who vowed that the UN would remain in Iraq to help restore peace and stability, told the Security Council he was surprised by reports that the world body had turned down an offer for increased protection around its Baghdad headquarter, which was blown up by a truck laden with explosives on Tuesday, killing seventeen people, including the UN envoy in Iraq, Sergio Vieira de Mello.

"It is those who have the responsibility for security and law and order, who have intelligence, determine what action is taken. I don't know if the UN did turn down an offer of protection. If it did, it was not correct," he said.

But he said even if the UN did turn down the offer, "they (the coalition) should not have allowed it".

"We all live in this city (New York). Nobody tells you if you want the police to patrol you neighbourhood. They make an assessment that patrol and protection is needed. And that's what should be done in Iraq," he later told reporters.

Nation building, Annan said, is a tough assignment. But "when you have this kind of terrorist attack and the people perpetrating the attack are prepared to die, providing security and protection is extremely difficult".

In a complex operation, he said, one has to plan. "I think there have been some wrong assumptions all along. The coalition has made some mistakes, and maybe we have made some mistakes too."

Asked to elaborate, Annan said, "I don't want to get into finger-pointing. But I think that we are all aware that along the way mistakes have been made by all concerned."

He said efforts are being made to bring the situation under control.

The international organisation, which is helping to rebuild the country, will not be deterred by the attack, he added.

"We will persevere; we will continue. It is essential work."