A bomb blast hit the United Nations building in the Nigerian capital of Abuja on Friday, the UN said, and witnesses said they had seen dead bodies being carried from the site.
"I can confirm there was an explosion at the UN building," a police spokesman in Abuja said.
"We have deployed our policemen and anti-bomb squad. We can't establish how many casualties [there are]." Witnesses said they saw ambulances carrying casualties from the site.
In Geneva, UN spokeswoman Alessandra Vellucci said an official at the UN information centre in Lagos, Nigeria's biggest city, had confirmed that a bomb was to blame.
She had no further information and it was not clear who was responsible for the attack.
Nigeria has been hit by a wave of bombings claimed by Boko Haram, a radical Islamist sect.
Boko Haram, whose name translates from the local northern Hausa language as "Western education is sinful," has been behind almost daily bombings and shootings, mostly targeting police in the northeast of Africa's most populous nation.
On Thursday it bombed a police station and raided banks in a northeastern Nigerian town, leaving 12 people dead, including policemen and a soldier.
"We just saw the blast coming from the building. All the people in the basement were all killed. Their bodies are littered all over the place. I saw about five dead bodies," said Ocilaje Michael, a member of the UN staff working at the building. A Reuters witness confirmed dead bodies being carried into an ambulance.