The sources at U.N. headquarters in New York told The (Lagos) Guardian it will not accept claims of responsibility by any group until the investigation has been completed.
Twenty-three people died and dozens were injured Friday when a suicide bomber rammed his car into the reception hall at U.N. headquarters in the Nigerian capital of Abuja.
The fundamentalist Islamic sect Boko Haram has claimed responsibility for the attack.
Officials, who spoke on condition of anonymity, said Secretary-General Ban Ki-moon has refused to comment until the United Nations "independently determines those behind the attack."
The United Nations has released the nationalities of some of those killed in the blast.
The organization said Tuesday 10 of the dead were Nigerians and one was a Norwegian.
Twenty-six people injured in the attack remain hospitalized.