At least 10 people have been killed in an apparent suicide car bombing at the UN building in the Nigerian capital Abuja.
The powerful blast wrecked the bottom floor of the building. Dozens have been injured, some critically.
A UN official in Nigeria, who spoke to the BBC on condition of anonymity, said the UN had received information last month that it could be targeted by Islamist group Boko Haram.
Security was stepped up in response.
A car bombing at police headquarters in June was blamed on Boko Haram, a group which wants the establishment of Sharia law in Nigeria.
There has been no claim of responsibility for Friday's attack, but government officials have blamed "terrorists" for the bombing, says the BBC's Bashir Sa'ad Abdullahi, who is at the site of the explosion.
Witnesses said the blast happened when a car rammed the front of the building after crashing through two security barriers.
Nigeria's Minister of State for Foreign Affairs, Viola Onwuliri, told the BBC she had visited the building and seen "the shock on people's faces".
"This is not an attack on Nigeria but on the global community," she said. "An attack on the world."
The building is the UN's main office in Nigeria, where all its agencies are based, the unnamed UN official told the BBC.
The official said security had been increased at all UN offices in Nigeria following the intelligence warning of a possible attack.
'Scattered bodies'
Friday's attack took place at about 1100 local time (1000 GMT) in the diplomatic zone in the centre of the city, close to the US embassy.
There was a loud explosion and smoke billowed from the building following Friday's powerful blast, which shattered all the glass in the building.
Our correspondent said the ground floor of the building was badly damaged and he saw the emergency services removing dead bodies from the building while a number of wounded were rushed to hospital.
One UN staff member said that people may still be trapped in the building, the AFP news agency reported.
"I saw scattered bodies," Michael Ofilaje, a Unicef worker at the building, said according to the Associated Press news agency (AP).
"Many people are dead."
He said it felt like "the blast came from the basement and shook the building".
Local hospitals are said to be overwhelmed with the number of injured and have appealed for blood donations.
Police have sealed off the area.
TT Owonubi