Witnesses told AP that a car rammed through two sets of gates protecting the compound and drove towards the main building.
The suicide bomber then detonated the bomb outside the main reception.
Reports on the number of dead range between seven and 18.
UN deputy spokesman Farhan Haq, speaking from New York, said: “There are some indications of fatalities, but we are trying to get confirmation.”
Locals are reporting an entire wing of the building has been destroyed, with an administrator of a nearby hospital saying 40 victims had so far been treated, but with more injured people arriving.
UN worker Ocilake Michael told Reuters: “We just saw the blast coming from the building. All the people in the basement were killed. Their bodies are littered all over the place”.
A Unicef worker, Michael Ofilaje, told AP: “The blast came from the basement and shook the building.”
He warned “I saw scattered bodies” and that “Many people are dead”.
There are reports that the wounded are being evacuated.
UN spokeswoman Alessandra Vellucci confirmed that its offices had been bombed, but couldn’t give any details on casualties.
The building houses around 400 employees, and houses most of the UN’s offices.
Police spokesman Jimoh Moshood has confirmed the explosion occurred before 11am local time, in a region of the capital that is home to the US embassy and a number of other diplomatic buildings.
