Abuja, Nigeria (CNN) -- At least 18 people were killed Friday in a bombing at a U.N. building in the Nigerian capital, a police spokesman said.
Police commissioner Michael Zuokumo told reporters eight people had been injured.
Rescue officials at the scene in Abuja helped to pull the dead and injured from the rubble. Bomb squad officers and other security teams were also sent to the scene, deputy police spokesman Yemi Ajayi said.
NATO chief Anders Fogh Rasmussen condemned the bombing in a letter sent to U.N. Secretary General Ban Ki-moon Saturday.
"I was appalled to hear about the terrible bomb attacks," the NATO secretary general wrote. "NATO strongly condemns these dreadful and heinous acts, which deliberately target U.N. staff who are devoting their lives to help others."
Nigerian President Goodluck Jonathan a day earlier described the car bomb as a "barbaric, senseless and cowardly attack."
A statement issued by his office said extra security had been ordered in the area around the capital and Nigeria would do everything it could to bring the perpetrators to justice.
U.N. Secretary-General Ban Ki-moon, addressing the media in New York Friday, said exact casualty figures were not yet known but are likely to be "considerable."
He condemned the car bombing but would not speculate on who might be responsible.
The attack targeted a building that housed 26 U.N. humanitarian and development agencies, he said, adding: "This was an assault on those who devote their lives to helping others."
Ban said he was sending the deputy secretary general and the chief of U.N. security to Abuja, where they would try to determine who is responsible.
A representative for the National Hospital said there was a shortage of blood to use in surgery. Hospital staff and relatives of the injured went to the blood bank to donate.
The hospital called in all doctors and nurses who were not working Friday to deal with the emergency, the spokeswoman said.
Yushau Shuaib, a spokesman for Nigeria's National Emergency Management Agency, said the driver of the vehicle used in the bombing had passed through the first and second gates of the compound before detonating the explosives by the building's reception area.
A witness outside the U.N. building at the time of the attack told CNN she had seen a white SUV drive quickly through the main gate of the compound. She then heard two explosions in quick succession, the second one much larger and followed by the shattering of glass.
Images from the scene showed severe damage to the bottom floors of the building, with windows blown in and debris scattered around.
People who were in the U.N. building told CNN that after the explosion, a wall fell on some people, causing several casualties.
U.K. Foreign Secretary William Hague said he was shocked by the attack.
"This brutal act has killed and injured many innocent people, and I utterly condemn those who have carried it out," he said.
"Around the world the U.N. works for peace, security and international cooperation and an attack on the U.N. is an attack on these principles."
Alessandra Vellucci at the United Nations offices in Geneva, Switzerland, confirmed that the "U.N. premises in Abuja have been bombed" but was unable to give more details.
The building is home to the main U.N missions operating in Nigeria and may have had as many as 400 people inside.
CNN's Alkasim Abdulkadir said the area was not very busy because it was a diplomatic district, with the Liberian and U.S. embassies nearby, but there were people working there.
A spokeswoman for the U.S. embassy in Abuja, Deborah MacLean, told CNN that there had been no damage to the embassy building, which is half a mile from the U.N. building.
Embassy personnel were initially told to be careful in case of further violence, she said.
Zoran Jovanovic, head of mission for the International Red Cross in Abuja, said the explosion was heard at 10:15 a.m.
The U.N. building is in a large compound that has five gates, Jovanovic said, adding that the area is also home to Western African embassies.
It is not yet clear who was responsible for the attack.
The Nigerian capital has experienced a series of bombings recently.
In June, a car blast killed at least five people at the police headquarters in the city. It detonated less than two minutes after the federal police chief entered the building, a security source said at the time.
A month later, at least three people died in an explosion near a church outside the capital. The number of casualties could have been higher if services had still been going on, according to a police spokesman.
In October, at least 12 people died when car bombs exploded as the nation celebrated 50 years of independence. The attacks near the Justice Ministry injured about 50.
The Movement for the Emancipation of the Niger Delta claimed responsibility for the October attack. MEND, an umbrella organization of several rebel groups, is battling the government for fairer distribution of oil wealth.
Recent attacks, including on the church and police station, may have been the work of a radical Muslim sect, Boko Haram, the nation's officials said this year. Boko Haram aims to enforce a strict version of Sharia law in the nation.
Africa's most populous nation is divided between a largely Christian south and a Muslim north.
CNN's Alkasim Abdulkadir, Umaro Djau, Christian Purefoy, Ben Brumfield, Stephanie Halasz, David Wilkinson and Claudia Rebaza contributed to this report.