ABUJA, Nigeria (AP) -- A large explosion struck the United Nations' main office in Nigeria's capital Friday, flattening one wing of the building and killing several people. A U.N. official in Geneva called it a bomb attack.

   ``I saw scattered bodies,'' said Michael Ofilaje, a UNICEF worker at the building. ``Many people are dead.''
   He said it felt like ``the blast came from the basement and shook the building.''

   The building houses about 400 employees of the U.N. in Nigeria, including the majority of its offices. A local U.N. spokesman declined to comment.
   Alessandra Vellucci, a spokeswoman for the U.N. office in Geneva, said the global body's offices in Abuja had been bombed.

   She told The Associated Press that there was no word yet on casualties.
   The building, located in the same neighborhood as the U.S. embassy and other diplomatic posts in Abuja, had a huge hole punched in it.

   Local police spokesman Jimoh Moshood confirmed the blast, but said police were still investigating the cause.

   No one immediately claimed responsibility for the blast, but oil-rich Nigeria faces terrorism threats on multiple fronts. Last year, a militant group from the country's crude-producing Niger Delta blew up car bombs in the capital during Nigeria's 50th independence anniversary ceremony, killing at least 12.

   Nigeria, a nation of 150 million, is split between a largely Christian south and Muslim north. In recent months, the country has faced an increasing threat from a radical Muslim sect called Boko Haram, which wants to implement a strict version of Shariah law in the nation. 

The sect has carried out assassinations and bombings, including the June car bombing of the national headquarters of Nigeria's federal police that killed at least two people.

   Earlier this month, the commander for U.S. military operations in Africa said Boko Haram may be trying to link with two al-Qaida-linked groups in other African countries to mount joint attacks in Nigeria.

   Gen. Carter Ham told AP on Aug. 17 during a visit to Nigeria that ``multiple sources'' indicate Boko Haram made contacts with al-Qaida in the Islamic Maghreb, which operates in northwest Africa, and with al-Shabab in Somalia.

   ``I think it would be the most dangerous thing to happen not only to the Africans, but to us as well,'' Carter said.