A car laden with explosives rammed through two gates and blew up at the United Nations' offices in Nigeria's capital Abuja today, killing at least 16 people and shattering part of the concrete structure.
UN Secretary-General Ban Ki-moon called it "an assault on those who devote their lives to helping others".
The brazen attack, carried out as the four-storey UN offices teemed with staff, comes as Africa's most populous nation faces the growing threat of both homegrown and international terrorism.
UN deputy spokesman Farhan Haq said: "We do not have any confirmation as yet who was responsible."
But suspicion fell on Boko Haram, the Muslim sect with reported links to al Qaida that wants to implement a strict version of Shariah law in the nation and is vehemently opposed to Western education and culture.
Witnesses said a sedan rammed through two separate gates at the UN compound as guards tried to stop the vehicle.
The suicide bomber inside crashed the car into the main reception area and detonated the explosives, inflicting the most damage possible, a spokesman for the Nigerian National Emergency Management Agency said.
"I saw scattered bodies," said Michael Ofilaje, a UNICEF worker at the building, which he said shook with the explosion. "Many people are dead."
The Nigerian Red Cross reported at least 16 people died in the attack, with at least 11 others injured.
Nigerian Health Minister Mohammad Ali Pate made a public appeal for blood donation on the widely listened-to BBC Hausa language service, saying there were at least 60 injured people at the National Hospital in Abuja.
The buildings, known as UN House, had offices for about 400 employees working for 26 UN humanitarian and development agencies. Authorities were still trying to account for everyone in the building at the time of the blast.
"We condemn this terrible act, utterly," secretary-general Ban told reporters at UN headquarters. "We do not yet have precise casualty figures but they are likely to be considerable. A number of people are dead; many more are wounded."
The building, in the same neighbourhood as the US embassy and other diplomatic posts in Abuja, houses offices of a number of UN agencies including the UN Development Programme, UNICEF and the UN Population Fund.
The explosion punched a huge hole in the building. Workers brought three large cranes to the site within hours of the attack, trying to pull away the concrete and rubble to find survivors. Others at the site stood around, stunned, as medical workers began carrying out what appeared to be the dead.
"This is getting out of hand," said a UN staffer who identified himself as Bodunrin. "If they can get into the UN House, they can reach anywhere."
Ali Tikko, who was in a building 100 yards from the site of the blast said: "I heard one big boom."
Local police spokesman Jimoh Moshood said police are investigating. In a statement, Nigerian President Goodluck Jonathan's office called the attack "barbaric, senseless and cowardly." The statement also promised to increase security in the nation's capital.
"President Jonathan reaffirms the federal government's total commitment to vigorously combat the incursion of all forms of terrorism into Nigeria, and wishes to reassure all Nigerians and the international community that his administration will spare no effort to bring the perpetrators to justice," the statement read.
The secretary-general said he was sending Deputy Secretary-General Asha-Rose Migiro and UN security chief Gregory Starr to Abuja immediately to meet with Nigerian authorities and to "respond to this emergency."
Oil-rich Nigeria faces terrorism threats on multiple fronts.
Nigeria, a nation of 150 million, is split between a largely Christian south and Muslim north. In recent months, the country has faced an increasing threat from Boko Haram.
The sect has carried out assassinations and bombings, including the June car bombing in Abuja of the national headquarters of Nigeria's federal police that killed at least two people.
Earlier this month, the commander for US military operations in Africa said Boko Haram may be trying to link with two al Qaida-linked groups in other African countries to mount joint attacks in Nigeria.
General Carter Ham said during a visit to Nigeria that "multiple sources" indicate Boko Haram made contacts with al Qaida in the Islamic Maghreb, which operates in north-west Africa, and with al-Shabab in Somalia.
"I think it would be the most dangerous thing to happen not only to the Africans, but to us as well," he said.
Last year, a militant group from the country's oil patch, the Niger Delta, blew up car bombs in the capital during Nigeria's 50th independence anniversary ceremony, killing at least 12.