The complex in Abuja where 26 UN agencies had staff was considered "well-defended," according to deputy UN spokesman Farhan Haq. But he added that tough security questions would have to be answered.
"The car got through a couple of gates which were defended by security guards. How that happened? How they got past security? We will need to determine how that was the case," Haq told reporters.
At least 18 people are said to have been killed in the attack.
Haq said there were "barricades and different gates" around the Aruba building.
"It is regarded as a well defended building and we will need to investigate how it was that the attackers managed to get past different levels of protection so that they could cause the damage that they did."
The head of UN security Greg Starr has already been sent to Nigeria to launch an inquiry.
Copyright AFP (Agence France-Presse), 2011