A senior UN official has admitted that French investigators were delayed for months before being allowed to interview the author of a report on alleged child abuse by peacekeepers in Africa. Speaking for the first time about allegations that French soldiers operating under a UN mandate sexually exploited boys in the Central African Republic, the UN high commissioner for human rights said: “We could have done better.”

Zeid Ra’ad Al Hussein said that on Friday he had met Anders Kompass, the UN human rights worker who leaked a report on the sexual abuse to the French authorities last year.

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Sources close to the case have told the Guardian that he disclosed the details to the French prosecutor because he wanted the abuse stopped quickly and feared that the UN would delay taking any action.

Nine months after passing on the report – which details interviews with children between eight and 15 – he was called in by Zeid and asked to resign. When he refused, Kompass was suspended while an investigation began into the leaking of confidential information. This week, however, an appeal tribunal ruled the UN’s actions unlawful and ordered Kompass to be reinstated.

The high commissioner said he had not apologised to Kompass – who was allowed to return to work on Wednesday.

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“I did meet Kompass today,” he said. “We had a long discussion. We agreed that there is no monopoly by any one person on the sense of outrage; we are all united in outrage. I didn’t apologise to Anders Kompass. I think it was regrettable that the burden of responsibility has almost entirely shifted on to the UN.”

He said the UN had accepted the judgment of the appeal tribunal that his suspension was unlawful – but added: “It doesn’t mean we accept all their reasoning.”

The Guardian revealed last week that Kompass had been suspended and faced dismissal for leaking the report detailing the alleged sexual abuse of children in CAR by French peacekeepers.

The French authorities began a preliminary investigation last August – after receiving the report – but only this week said they were mounting a criminal investigation.

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They cited more than six months of delays in French investigators’ efforts to get answers from the author, a UN staff member, which finally resulted in written responses received on 29 April.

Zeid admitted the delay at a press conference in Geneva on Friday. He said: “The whole operation was a long operation, and its very much needed that all of us – whether it is the UN or member states – think about shortening the timeline. We could have done better in terms of the time it took.”

He said the alleged abuse was likely the tip of a much bigger problem. But he blamed the French – who had jurisdiction over the soldiers – for not responding sooner to the allegations. The internal UN report contained interviews with six children alleging sexual abuse from December 2013 to May 2014. Zeid said: “What was happening in December, in March, in May? Someone knew, they didn’t report. Who was that person?”

The French said this week they were investigating “the rape of minors under 15 years old by persons who had abused the authority conferred upon them by their roles, and complicity in this crime”. “Investigations will now continue under the authority of an instructing magistrate in order to get to the truth of the accusations,” a statement from French prosecutors said.