Witnesses reported that the bomb went off after a suspect forced his way through security and rammed the car into the building. Parts of the first two floors were blown out and rescue workers scrambled to rescue those left inside.
There was no immediate claim of responsibility, but an Islamist sect known as Boko Haram has been blamed for scores of bombings in recent months.
"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."
UN chief Ban Ki-moon strongly condemned the attack and said "considerable" casualties were expected. Ban said staff for 26 UN agencies and departments were in the building.
"This was an assault on those who devote their lives to helping others," he said. "We condemn this terrible act utterly."
A Security Council meeting on peacekeeping around the world started with one minute`s silence in respect for the victims.
Ban, who was at the building two months ago, said he was sending Deputy UN Secretary General Asha-Rose Migiro and UN security chief Gregory Starr to Nigeria immediately.
He added that he would soon be talking with Nigeria`s President Goodluck Jonathan, who also condemned the attack and pledged that authorities would hunt the perpetrators.
A member of security personnel speaking on condition of anonymity spoke of "many dead".
AFP correspondents saw wounded people being taken from the building, including those with bloodied heads. Some appeared lifeless but it was unclear whether they were dead.
One UN staff member said people had been 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," the UN staffer who did not want to give her name said in the aftermath of the explosion.
Two cranes were later brought to the scene and rescue workers sought to free those trapped on the upper floors.
The UN building is located in Abuja`s diplomatic zone, not far from the US embassy.
Security is usually extremely tight, with non-UN vehicles typically not allowed to approach the gate leading to the compound, and the building is set back from the street.
A bomb blast that rocked a car park at national police headquarters in Abuja in June and killed at least two people was claimed by Boko Haram. Police first said it was the result of a suicide blast before later retracting their statement, saying they could not be sure.
Most of the attacks blamed on the sect have occurred in the country`s northeast, but a number have been carried out elsewhere, including the previous explosion in Abuja as well as several in Suleija near the capital.
The sect launched an uprising in 2009, put down by a brutal military assault that left hundreds dead. It went dormant for about a year before reemerging in 2010 with a series of assassinations of security personnel and politicians, as well as religious and community leaders.
There have been growing concerns that the sect has formed links with extremist groups outside of Nigeria, including Al-Qaeda`s north African branch. (*)
Editor: Kunto Wibisono
Copyright © ANTARA 2011
