A car bomb was detonated at the UN offices in Abuja, the Nigerian capital, with reports suggesting as many as 16 people may be dead.
Other survivors are also allegedly trapped as the blast was so powerful it blew out large areas of the building, officials said
Witnesses told AP that a car rammed through two sets of gates of the compound and drove towards the main building before a suicide bomber detonated the bomb near the main reception area.
As the number of casualties is still uncertain, UN deputy spokesman Farhan Haq, speaking from New York, said: "There are some indications of fatalities, but we are trying to get confirmation."
Reports suggest that the blast destroyed an entire wing of the building and staff from a neaby hospital told the press up to 40 victims had so far been treated, before adding that more injured people arriving.
"We just saw the blast coming from the building. All the people in the basement were killed. Their bodies are littered all over the place," UN worker Ocilake Michael told Reuters
A Unicef worker, Michael Ofilaje, told AP: "The blast came from the basement and shook the building," adding "I saw scattered bodies" and that "Many people are dead".
UN spokeswoman Alessandra Vellucci confirmed that its offices, where up to 400 people work had been bombed, but did not provide further details on casualties.
The UN offices are located in an area of the capital that also houses other diplomatic buildings such as the US embassy.
No groups have yet claimed responsibility for the attack, but in June, the radical Muslim group called Boko Haram or "Western education is sinful", carried out a bombing at Nigeria's federal police headquarters that killed two people and last week also claimed responsibility for a failed bomb attack in Borno State.
Highlighting the development of Boko Haram, a U.S. official warned that the group is now rumoured to have close ties with other terrorists groups such as al-Qaeda in the Islamic Maghreb and the Somali Muslim extremist group al-Shabab.
Security in Nigeria still remains volatile with many Islamists group emerging and A UN worker, told AP: "This is getting out of hand. If they can get into the UN House, they can reach anywhere."