Witnesses told The Associated Press that a sedan broke through the exit at the U.N. compound, ramming through two separate gates as guards tried to stop the vehicle. The suicide bomber inside drove the car just up to the main reception of the building before detonating, inflicting the most damage possible, witnesses said.
"I saw scattered bodies," said Michael Ofilaje, a UNICEF worker at the building. "Many people are dead."
He said it felt like "the blast came from the basement and shook the building."
The building houses about 400 employees of the U.N. in Nigeria, including the majority of its offices. A local U.N. spokesman declined to comment, but a local hospital administrator told the AP it had treated as many as 40 victims so far, with more people coming in.
Alessandra Vellucci, a spokeswoman for the U.N. office in Geneva, confirmed that the global body's offices in Abuja had been bombed.
She told the AP that there was no word yet on casualties.
U.N. deputy spokesman Farhan Haq in New York said, "There are some indications of fatalities, but we are trying to get confirmation."
The building, located in the same neighborhood as the U.S. embassy and other diplomatic posts in Abuja, had a huge hole punched in it. Workers brought three large cranes to the building by midday Friday, trying to pull away the concrete and rubble to try and 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 U.N. staffer who identified himself as Bodunrin. "If they can get into the U.N. House, they can reach anywhere."
Ali Tikko, who was in a building 100 yards (meters) from the site of the blast when it occurred said, "I heard one big boom." Outside his window, he said, he could see a part of the damage.
"I see a number of people lying on the floor - at least four or five. I cannot see if they are dead. There are a lot of security around," said Tikko who was reached by telephone Friday morning.
Local police spokesman Jimoh Moshood confirmed the blast, but said police were still investigating the cause. Reuben Abati, a spokesman for President Goodluck Jonathan, said the presidency would later issue a statement on the attack.
No one immediately claimed responsibility for the blast, but oil-rich Nigeria faces terrorism threats on multiple fronts. Last year, a militant group from the country's crude-producing Niger Delta blew up car bombs in the capital during Nigeria's 50th independence anniversary ceremony, killing at least 12.
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 a radical Muslim sect called Boko Haram, which wants to implement a strict version of Shariah law in the nation. The sect has carried out assassinations and bombings, including the June car bombing of the national headquarters of Nigeria's federal police that killed at least two people.
Earlier this month, the commander for U.S. 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.
Gen. Carter Ham told AP on Aug. 17 during a visit to Nigeria that "multiple sources" indicate Boko Haram made contacts with al-Qaida in the Islamic Maghreb, which operates in northwest 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," Carter said.
Associated Press writers Jon Gambrell and Yinka Ibukun in Lagos, Nigeria, Michelle Faul in Johannesburg, Rukmini Callimachi in Dakar, Senegal and Frank Jordans and John Heilprin in Geneva contributed to this report.