A bomb blast at the United Nations building in Nigeria's capital, Abuja, has killed at least 16 people.

Witnesses report seeing a vehicle force its way past security and explode once it reached the four-story building on Friday morning.

Rescuers raced to pull bodies and survivors from the rubble at the compound.

Speaking from New York, U.N. Secretary-General Ban Ki-moon predicted the casualty toll will be “considerable,” but added he did not yet have an exact number.

Mr. Ban called the attack a “terrible act” against people who devote their lives to helping others. He said the U.N. compound houses 26 humanitarian and development agencies. An estimated 400 people work at the complex.

The secretary-general said a U.N. team led by Deputy Secretary-General Asha-Rose Migiro will travel to Abuja to assess the situation.

He declined to speculate on a motive for the attack, and there was no immediate claim of responsibility.

Police have blamed a string of bombings in the country on the radical Islamic sect Boko Haram. The group has called for sharia, a form of strict Islamic law it would like applied more widely and strictly across Nigeria.

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

It 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.