The United Nations' special envoy to Iraq Sergio Vieira de Mello was among 18 persons killed in a powerful truck bomb explosion at its headquarters in Baghdad, the world body announced in New York on Tuesday.

The truck bomb exploded just outside the parameter of the headquarters but then the parameter is very near the building, the UN spokesman said.

"I saw 13 dead in front of me," UN spokesperson Veronique Taveau told a news agency by phone. Over 50 persons are believed injured in the attack. They have been admitted to hospitals around Baghdad.

The UN staff works in dangerous conditions several times and officials said that those who join the mission know that they are doing so at some risk to themselves, UN officials said adding there have been several attacks against the UN and its workers in the past, but the Baghdad attack was unmatched in ferocity.

No one has so far claimed responsibility for the attack.

Officials say security might not have been very tight as the UN does not want its headquarters to appear like a fortress. The security at the UN headquarters and also other premises was tightened following the attack.

The UN Security Council condemned 'in the strongest' terms the attack and asserted such incidents cannot break the 'will of the international community' to  further intensify its efforts to help the people in Iraq.

"Members of the council were shocked to hear of the terrorist, criminal attack," the Syrian president of the Security Council said in a statement.

The members, in a statement, reaffirmed that 'this horrible' attack aimed at undermining the 'vital role' of the United Nations in Iraq will not affect their determination and they will remain united against such attacks.

The members also expressed condolence to the bereaved families and wished the injured a quick recovery. The UN lowered its flag to half-mast to mourn the death of its staffers.

Secretary-General Kofi Annan, vacationing in northern Europe, cut short his holiday and headed back to the UN headquarters in New York in view of the tragedy, which shocked the staff at the headquarters in New York.

There are about 600 foreign UN personnel in Iraq, of which 300 are in Baghdad. An equal number of locals are working for the UN.

Agencies