At least 18 people killed were killed when a car bomb exploded on Friday at the UN's headquarters in Abuja, Nigeria.
The recent attack has increased fears that a local Islamist militant group inspired by the Taliban may be widening its terror attacks to include Western targets.
Boko Haram, a radical Islamist sect, has reportedly claimed responsibility for the attack.
The attack is said to be the most serious carried out by the group, which is based in Nigeria's Muslim majority north, according to the Los Angeles Times. Some believe the group is forging ties with al Qaeda, the Times report stated.
More attacks are on the way, and by the will of Allah we will have unfettered access to wherever we want to attack, spokesman Abu Darda, said by phone, as reported by the Times. We have more than 100 men who are willing to lay down their lives for the cause of Allah.
U.N. Secretary-General Ban Ki-moon has condemned Friday's attack and said he is dispatching top officials to the Nigerian capital to respond to the emergency.
The compound in Abuja is a series of buildings that house the offices of 26 UN humanitarian and development agencies. It was struck by a car bomb at around 11 a.m. local time. Hundreds of UN staff members were working in the compound at the time of the attack, the UN said.
This was an assault on those who devote their lives to helping others, Ban told reporters at UN Headquarters in New York City. We condemn this terrible act, utterly.