At least 18 people have been killed in an apparent suicide car bombing at the United Nations headquarters in the Nigerian capital, Abuja.

A bomb blast ripped through the UN offices as a car rammed into the building, and witnesses said they had seen a number of dead bodies being carried from the site.

The powerful blast destroyed the lower floors of the building, and it is understood that in addition to the dead dozens have been injured, some critically.

A hospital in Abuja said at least 60 people had been brought in, many in critical condition.

The Department of Foreign Affairs says all Australians registered in Abuja are safe, and staff at the Australian High Commission were not caught up in the attack.

One security source in Abuja said he suspected an attack either by Boko Haram - a Nigerian radical Islamist sect - or the North African arm of al Qaeda.

The BBC reported that a spokesman for Boko Haram had said in a phone call that it had carried out the attack. The BBC gave no further details.

It is difficult to get confirmation of attacks by Boko Haram because the group has an ill-defined command structure and a variety of people who speak on its behalf. The police and the government have not said who was responsible.

The UN building was blackened from top to bottom and the remains of a car had fallen into the basement. Soldiers, firefighters and rescue workers swarmed over the area.

Nigeria's president Goodluck Jonathan condemned the attack and vowed to hunt down those responsible.

UN leader Ban Ki-moon said the attack highlighted how the global body was increasingly becoming a "soft target" for extremists.

Mr Ban strongly condemned the attack and said a security review would be carried out, adding that 26 agencies were in the UN compound and that "considerable" casualties were expected.

"This was an assault on those who devote their lives to helping others. We condemn this terrible act utterly," Mr Ban told reporters.

He then attended a Security Council meeting on peacekeeping around the world, which started with one minute's silence for the victims and released a statement "condemning the attack in the strongest terms."

The UN leader highlighted his fears of more attacks.

"Let me say it clearly, these acts of terrorism are unacceptable, they will not deter us from our vital work for the people of Nigeria and the world," he told the 15-nation council.

"This outrageous and shocking attack is evidence that the UN premises are increasingly being viewed as a soft target by extremist elements around the world."

ABC/wires