Nigeria's president has visited the United Nations headquarters in the capital, Abuja, where a car bombing killed at least 19 people.
President Goodluck Jonathan offered condolences Saturday to U.N. Secretary-General Ban Ki-moon for what he called a "dastardly" act. He said his government is working to bring terrorism "under control," and said it would also work with the United Nations and other world leaders to respond to the attack.
U.S. officials said Saturday that agents from the FBI arrived to help the Nigerian government with the investigation.
Nigeria's government has tightened security in Abuja, with soldiers searching cars at roadblocks and patrolling around the damaged building.
Authorities say the death toll could still rise as hospitals continue to treat at least 60 people wounded in the blast. About 400 people work at the U.N. compound in Abuja, which houses 26 humanitarian and development agencies.
Witnesses say a vehicle forced its way past two security gates at the sprawling U.N. complex shortly before noon Friday, local time, and exploded inside the compound.
A man who identified himself as a spokesman for Boko Haram, an extremist Islamic group in Nigeria, said the group is responsible for the bombing. He told a VOA correspondent with the Hausa Service in Nigeria "this is just the beginning."
The spokesman said the bombing was in response to the military's increased presence in northeastern Borno state, where Boko Haram is very active.
Ban told a meeting of the Security Council Friday such "acts of terrorism are unacceptable." He 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.
Boko Haram has mostly carried out attacks in Nigeria's northeast, but has also claimed responsibility for several attacks in other regions, including the June bombing of Nigeria's national police headquarters in Abuja.
The group wants a strict form of Islamic law called sharia to be more widely imposed across Africa's most populous nation.
Boko Haram launched a violent uprising in July 2009 that was crushed by the Nigerian military. Since then, the group has targeted police, politicians, community leaders and opposition religious figures.
Some information for this report was provided by AP, AFP and Reuters.