The building is in the same neighborhood as the U.S. embassy.
The BBC adds that there's been no claim of responsibilty (and it is still not certain that it was a bombing), but also notes that "Islamist militants have carried out recent attacks on the city."
We'll update this post as the story develops.
Update at 12:35 p.m. ET. Suicide Bomber:
According to The New York Times, "a suicide bomber driving a vehicle packed with explosives" was responsible. It adds that:
"Boko Haram, a shadowy Nigerian Islamist insurgency group with possible links to Al Qaeda's affiliates in the region, claimed responsibility for the attack in a telephone call to the BBC's Hausa language broadcast service in northern Nigeria."
Update at 9:45 a.m. ET. Death Estimates:
Reuters is reporting that it has been told by U.N. officials that more than 16 people were killed. The BBC says the number of deaths is "at least 10." Dozens of people may have been wounded.
Update at 8:10 a.m. ET: NPR's Ofeibea Quist-Arcton, who is in Kenya reporting on the drought and famine in the Horn of Africa, is monitoring the news. She tells us that "witnesses outside the U.N. building say the blast happened at mid-morning."