A car loaded with explosives crashed into the main United Nations' building in Nigeria's capital and exploded Friday, killing at least 18 people in one of the deadliest assaults on the international body in a decade. A radical Muslim sect blamed for a series of attacks in the country claimed responsibility for the bombing, a major escalation of their sectarian fight against Nigeria's weak central government.

The brazen assault in a neighborhood surrounded by heavily fortified diplomatic posts represented the first suicide attack to target foreigners in oil-rich Nigeria, where locals already live in fear of the radical Boko Haram sect. The group, which has reported links to al-Qaida, wants to implement a strict version of Shariah law in the nation and is vehemently opposed to Western education and culture.

While police officers and local officials have primarily bore the brunt of Boko Haram's rage, now everyone seems to be a target in a nation often divided by religion and ethnicity.

"It is an attack on the global community," said Viola Onwuliri, a junior Nigerian foreign minister, as she looked at the bomb site.

A sedan loaded with explosives crashed through two gates at the exit of the United Nations compound Friday morning as guards tried in vain to stop it, witnesses told The Associated Press. The suicide bomber inside drove the car through the glass front of the main reception area of the building and detonated the explosives, inflicting the most damage possible, a spokesman for the Nigerian National Emergency Management Agency said.

"I saw scattered bodies," said Michael Ofilaje, a UNICEF worker at the four-story building, which he said shook with the explosion. "Many people are dead."

At least 18 people died in the attack, according to an AP survey of morgues at four major Abuja hospitals. Nigerian Health Minister Mohammad Ali Pate made a public appeal for blood donations, saying there were at least 60 injured people alone at the nearby National Hospital.

The headquarters, known as U.N. House, had offices for about 400 employees working for 26 U.N. humanitarian and development agencies. Authorities worked Friday to account for everyone in the building at the time of the blast.

U.N. Secretary-General Ban Ki-moon called the car bombing "an assault on those who devote their lives to helping others."

"We condemn this terrible act, utterly," Ban told reporters at U.N. headquarters. "We do not yet have precise casualty figures but they are likely to be considerable. A number of people are dead; many more are wounded."

Said Djinnit, the special representative of the U.N. secretary-general for West Africa, told the AP that he expects the casualties are mostly local staffers because the offices where his U.N. colleagues work are not located on the ground floor which took the brunt of the blow.

The attack was one of the deadliest attacks on the United Nations in a decade. Seventeen U.N. civilian staff members were killed along with dozens of others in two terrorist car bombings that targeted U.N. and other premises in Algiers on Dec. 11, 2007. Friday's bombing also came just days after the U.N. marked the eighth anniversary of the Aug. 19, 2003 bombing of U.N. headquarters in Baghdad that killed 15 U.N. staff including top envoy Sergio Vieira de Mello and seven others.

The attack was also condemned by leaders around the world and members of the U.N. Security Council who individually deplored the targeting of the U.N. at an open meeting on U.N. peacekeeping.

U.S. President Barack Obama called the attack "horrific and cowardly" and expressed strong support for the U.N.'s work.

"The people who serve the United Nations do so with a simple purpose: to try to improve the lives of their neighbors and promote the values on which the U.N. was founded _ dignity, freedom, security, and peace," Obama said in a statement. "An attack on Nigerian and international public servants demonstrates the bankruptcy of the ideology that led to this heinous action."

The explosion punched a huge hole in the building, located in the same neighborhood as the U.S. embassy and other diplomatic posts in Abuja. Workers brought three large cranes to the site within hours of the attack, trying to pull away the concrete and rubble to find survivors. Others at the site stood around, stunned, as medical workers began carrying out what appeared to be the dead.

"This is getting out of hand," said a U.N. staffer who identified himself as Bodunrin. "If they can get into the U.N. House, they can reach anywhere."

Local police spokesman Jimoh Moshood said detectives had begun an investigation. In a statement, Nigerian President Goodluck Jonathan's office called the attack "barbaric, senseless and cowardly." The statement also promised to increase security in the nation's capital.

However, Jonathan's administration has struggled to improve security in Nigeria, a nation of 150 million largely split between a Christian south and Muslim north. The Christian president's election in April brought religious and ethnic violence across the north that left 800 people dead.

A spokesman for Boko Haram claimed responsibility for the attack later Friday in a communique to the BBC's Hausa language shortwave radio service, which is widely trusted and listened to throughout Nigeria's Muslim north. The sect has made such claims before to the service.

Boko Haram, which means "Western education is sacrilege," has carried out a series of bombings and assassinations in northern Nigeria in the last year. They claimed a car bombing that struck Nigeria's federal police headquarters in June that killed at least two people.

The sect came to national prominence in 2009, when its members rioted and burned police stations near its base of Maiduguri, a dusty northeastern city on the cusp of the Sahara Desert. Nigeria's military violently put down the attack, crushing the sect's mosque into shards as its leader was arrested and died in police custody. About 700 people died during the violence.

Sect members are scattered throughout northern Nigeria and nearby Cameroon, Chad and Niger. In the last year, they've unleashed a series of targeted killings and bombings.

But attacking foreigners is a new, troubling step for the group. Earlier this month, the commander for U.S. military operations in Africa told the AP that Boko Haram may be trying to coordinate attacks with two al-Qaida-linked groups _ 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," Gen. Carter Ham said Aug. 17.

The attack Friday shows Boko Haram has aspirations beyond targeting local government officials, said Innocent Chukwuma, a Nigerian criminologist and director of a police reform organization.

"Today's has taken it to the international level," Chukwuma said. "The choice of target shows we are perhaps dealing with a group well-connected to international networks."

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Gambrell reported from Lagos, Nigeria. Associated Press writers Yinka Ibukun in Lagos, Nigeria; Njadvara Musa in Maiduguri, Nigeria; Michelle Faul in Johannesburg; Rukmini Callimachi in Dakar, Senegal; Edith M. Lederer in New York and Frank Jordans and John Heilprin in Geneva contributed to this report.

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Jon Gambrell can be reached at http://www.twitter.com/jongambrellAP