The UN secretary-general says he fears his organisation has become a soft target, after a bomb attack on its Nigerian headquarters.
Witnesses reported seeing a car packed with explosives rammed through two separate gates, exploding just outside the building's main reception.
The suicide attack in Abuja killed 18 people.
The UN compound houses about 400 employees and the bulk of its offices in Nigeria.
No one has yet claimed responsibility for the attack.
However, the Nigerian police say they are investigating to see if the Islamist group Boko Haram might be behind the bombing.
The group wants Islamic Sharia law implemented in the country and has recently launched several attacks.
But violence in the past has centred on Nigerian symbols of authority. Not international organisations like the UN. Which makes this attack so much more worrying for people.
Al Jazeera's Yvonne Ndege reports from the Nigerian capital.