The UN has not been officially informed of arrests over last week's attack on its headquarters in Nigeria, an official said Thursday, after the country's authorities claimed two suspects were held.

"Let me tell you that the UN has not been informed officially of any arrest of any form," UN resident coordinator Daouda Toure told journalists. "We demand that there is no impunity."

Nigeria's secret police said Wednesday that an Al-Qaeda-linked suspect who recently returned from Somalia masterminded last week's attack in Abuja that killed at least 23 people.

It also said he worked in concert with two other suspects, identified as key figures of the Boko Haram extremist sect, who were arrested on August 21, days before the UN bombing.

Nigerian authorities said they were being held at a military facility, while the Al-Qaeda-linked suspect, Mamman Nur, was being sought.

Friday's attack was one of the bloodiest targeting the UN and saw the suicide bomber make his way past two gates before slamming his car into the entrance of the building.