The Baghdad bomb attack which left at least 17 people dead - including one British woman - and scores more wounded at the UN's base has been condemned by the British Government.
Rescuers are trying to reach those trapped in the rubble
Among the dead was Fiona Watson, a Briton on the staff of the head of the UN mission to Iraq, Sergio Vieira de
Mello, who was also killed.
Foreign Secretary Jack Straw said the dead envoy was an "exceptional man".
Mr Straw, who met Mr de Mello in Baghdad in July, described his death as an "utter tragedy".
The massive explosion had been a "totally unjustified attack" against the Iraqi people and the wider international community, the foreign secretary said.
He said the blast was clearly the work of "very, very ruthless people" of the type who had sustained Saddam Hussein's regime.
But responding to suggestions that terror group al-Qaeda may have been involved, Mr Straw said that it was too early to speculate about who exactly was to blame.
Mr Straw said: "My thoughts are with the relatives and friends of those who have been killed and
He was struck by the "complete dedication and commitment to the reconstruction of Iraq" of Mr de Mello.
Despite the shock of the bombing, Mr Straw said it would not deter the work to rebuild Iraq and would serve to strengthen the determination of those involved.
He said: "It's a serious matter, but it's not going to set back either the resolve of the coalition of the United States and the United Kingdom... nor that of the United Nations."
The foreign secretary said the attack was made all the more appalling as United Nations staff were in Baghdad "simply for the people of Iraq, to meet their immediate humanitarian needs".
Jack Straw said Britain would help find those responsible
The aim was "to assist the people of Iraq in rebuilding their country from the ravages of Saddam Hussein", Mr Straw said.
The foreign secretary said Britain would join efforts to find those responsible.
Mr Straw said he would be speaking to UN Secretary General Kofi Annan about the attack.
In a statement from his holiday in Barbados, Prime Minister Tony Blair said: "We will not allow terrorists to weaken our resolve in bringing about a better Iraq.
"Their evil only serves to reiterate the importance of our presence in the country and the action we have taken
to remove the regime of Saddam Hussein.
"The perpetrators of the atrocity have demonstrated pure cowardice and are enemies not only of the UN and coalition but also of the Iraqi people."
His outrage at the attack was echoed by other members of the international community.
Nick Roseveare, Oxfam's acting humanitarian director, said he was "appalled" at Mr de Mello's death.
He said: "Sergio Vieira de Mello was one of the most passionate, authoritative and influential protectors and advocates of humanitarian ideals."
A UN spokesman in Baghdad, Salim Lone, called the explosion "an unspeakable crime against people from all over the world who have come here to help the people of Iraq".
US President George W Bush said: "The civilised world will not be intimidated, and these killers will not determine the future of Iraq. Iraq is on an irreversible course towards self-government and peace."