At least 18 people have been killed in an apparent suicide car bombing at the United Nations headquarters in the Nigerian capital, Abuja.
The powerful blast destroyed the lower floors of the building. Dozens have been injured, some critically.
A spokesman for the Islamist group Boko Haram told the BBC in a phone call that it had carried out the attack.
UN chief Ban Ki-moon said the attack was "an assault on those who devote their lives to helping others".
Speaking before Boko Haram's statement, President Goodluck Jonathan reaffirmed his government's "total commitment" to combating terrorism, and said his administration would "spare no effort to bring the perpetrators to justice".
In Friday morning's attack, a car crashed through two security barriers and rammed into the building's reception before exploding, witnesses said.
Hospitals are said to be overwhelmed by the number of injured and have appealed for blood donations.
Boko Haram, which is fighting for the establishment of Sharia law in Nigeria, also said it carried out a car bombing at police headquarters in June.
A UN official in Nigeria, who spoke to the BBC on condition of anonymity, said the UN had stepped up security at all its buildings in Nigeria in the past month after receiving information that the UN could be targeted by Boko Haram.
Analysts say that the scale and target of the attack could point to a link with international terror groups.
'Shock on faces'
Police have sealed off the area and the BBC's Bashir Sa'ad Abdullahi there says that although rescue work is ongoing, the major search operation has been completed.
Cranes have been brought to the blast site to move the mass of rubble and ensure that no-one is trapped there.
"We condemn this terrible act utterly," UN Secretary General Ban Ki-moon told a news conference.
He said he was sending two deputies - Deputy Secretary General Asha-Rose Migiro and UN security chief Gregory Starr - immediately to meet officials in Nigeria.
Later, Mr Ban reportedly told a Security Council meeting that the attack was "evidence that the UN premises are increasingly being viewed as a soft target by extremist elements around the world".
In a phonecall with Mr Ban and Mr Jonathan, British Prime Minister David Cameron described it as an "appalling attack" and passed on his condolelences, a statement said.
Nigeria's Minister of State for Foreign Affairs Viola Onwuliri told the BBC she had visited the building and seen "the shock on people's faces".
"This is not an attack on Nigeria but on the global community," she said. "An attack on the world."
The building is the UN's main office in Nigeria, where 26 humanitarian and development agencies are based.
It is said to be the headquarters for about 400 UN employees but it is not clear how many were inside the building at the time of the attack.
Earlier this month the most senior American general working on Africa, Gen Carter Ham, said many sources indicated ties between Boko Haram and al-Qaeda in the Islamic Magreb, which operates in North Africa, as well as the al-Shabab movement in Somalia, says the BBC's Africa analyst Martin Plaut.
These ties, if correct, would have provided Boko Haram with the expertise it needed to carry out these deadly attacks, he says.
Friday's attack took place at about 1100 local time (1000 GMT) in the diplomatic zone in the centre of the city, close to the US embassy.
There was a loud explosion and smoke billowed from the building following the powerful blast, which shattered all the glass in the building and demolished part of the concrete wall.
A wing of the building collapsed and the ground floor of the building was badly damaged. Emergency services were quickly on the scene removing dead bodies from the building and rushing the wounded to hospital.
"When the car got inside it went straight to the basement and exploded, killing people in reception, right and left," Abuja resident James John, who witnessed the attack, told Reuters.
"I saw scattered bodies," Michael Ofilaje, a Unicef worker at the building, said according to the Associated Press news agency (AP). "Many people are dead."