Witnesses reported that the bomb went off after a suspect rammed a car through the front gate. Parts of the first two floors of the building were blown out and rescue workers scrambled to extricate those trapped inside.

There was no immediate claim of responsibility, but a religious sect known as Boko Haram has been blamed for scores of bombings in recent months.

"This is very likely the work of Boko Haram and, or, AQIM (Al-Qaeda in the Islamic Maghreb) and is a serious escalation in

the security situation in Nigeria," A security source said. "This is the worst thing that could have happened."

"So far, we have 18 dead and eight injured," Mike Zuokumor, police commissioner for the Federal Capital Territory, which includes Abuja, told journalists. "It was a Honda Accord car. The suicide bomber died immediately as the bomb cut him into three. I cannot say how many people are still in the building. The rescue operation is still on."

A member of security personnel speaking on condition of anonymity spoke of "many dead." "A guy drove a Honda car, forced his way through the gate and rammed into the building, and then the bomb exploded," the security source said at the scene.

"When the car got inside it went straight to the basement and exploded, killing people in reception, right and left," said Abuja resident James John, who witnessed the attack. "The entire building, from the ground floor to the topmost, was just fire and smoke. I saw six bodies being carried out. I can't believe it."

One UN staff member said people were still trapped in the building that sustained heavy damage. "I don't know what is going on. Many people are still trapped upstairs and we need a crane to bring people down," said the UN staffer who did not want to give her name.

Two cranes were later brought to the scene and rescue workers were trying to free those trapped on the upper floors.

The UN building is located in Abuja's diplomatic zone, not far from the US Embassy, and houses a number of United Nations agencies. Security is usually extremely tight, with non-UN vehicles typically not allowed to approach the gate leading to the compound.

UN Secretary-General Ban Ki-moon condemned the attack and said the casualty toll was likely to be "considerable" although he had no exact figures. He said it was "an assault on those who devote their lives to helping others." In a statement to reporters, he said he was sending his deputy, Asha-Rose Migiro, to Nigeria immediately to meet officials in Abuja. Migiro, who is Tanzanian, will be accompanied by UN security chief Gregory Starr.