An extremist Islamic group in Nigeria says it carried out a car bombing that killed at least 18 people at the United Nations building in the capital, Abuja.

Witnesses say a vehicle forced its way past security gates at the sprawling complex and exploded inside the compound shortly before noon Friday local time.

After the attack, a Boko Haram spokesman telephoned a VOA correspondent in Nigeria, claiming his group carried out the bombing. He also warned “this is just the beginning.”

The spokesman said the bombing was in response to the military's increased presence in northeastern Borno state, where the Boko Haram is very active.

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.

U.S. President Barack Obama condemned the bombing, calling it a “horrific and cowardly attack.” He also reaffirmed U.S. support for the United Nations and the people of Nigeria.

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.