A radical Islamic group in Nigeria says it carried out a car bombing that killed at least 18 people at the United Nations building in Abuja.

Witnesses said a vehicle forced its way past security gates at the sprawling complex and exploded inside the compound at about 11 a.m. Friday .

Rescuers raced to pull bodies and survivors from the rubble.

A spokesman for the radical group Boko Haram telephoned a VOA reporter (Hausa service) in Nigeria and said the bombing “is just the beginning.”

In New York, U.N. Secretary-General Ban Ki-moon said he did not have exact casualty figures but predicted the toll would be “considerable.”

Mr. Ban told a meeting of the Security Council such “acts of terrorism are unacceptable,” and he warned the bombing signals that militants around the world increasingly view U.N. sites as “soft targets.”

About 400 people work at the U.N. compound in Abuja, which houses 26 humanitarian and development agencies.

The secretary-general said a U.N. team will travel to the Nigerian capital to assess the situation. He declined to speculate on who may have carried out the bombing.

In a written statement, Nigerian President Goodluck Jonathan called the bombing “barbaric, senseless and cowardly.” He reaffirmed the country's commitment to combating terrorism and vowed to hold the attackers accountable.

Nigerian police have blamed Boko Haram for a string of bombings in recent months. The group has said it was behind a major attack on Abuja's police headquarters in June.

Boko Haram wants a strict form of Islamic law called sharia to be more widely imposed across Africa's most populous nation.

The group has targeted police, politicians, community leaders and opposition religious figures.

Boko Haram launched a violent uprising in July 2009 that was crushed by the Nigerian military. Since then, the group has targeted authority figures in shootings and carried out numerous bombings.