A radical Islamist group has claimed responsibility for a deadly suicide car bomb attack on the UN compound in the Nigerian capital, Abuja.

The blast left at least 18 people dead and more trapped inside.

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.

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.

'I cannot say how many people are still in the building. The rescue operation is still on', he added.

UN chief Ban Ki-moon strongly condemned the attack and said 'considerable' casualties were expected.

Mr 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.'

He said the attack highlighted how the global body was increasingly becoming a 'soft target' for extremists.

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.