A senior UN official who went on hunger strike in support of child victims of the Israel-Hamas conflict has said he was assaulted by UN security guards at the organisation’s New York headquarters.

Bruno Donat, head of the UN Mine Action Service in Geneva, alleged the guards pushed him back on to the pavement outside the headquarters on Monday so forcibly that he fell, striking the back of his head. Donat did not break any bones and has since been discharged from hospital, but says he is still in intense pain.

The UN denied Donat was assaulted and claims he fell when he was prevented from forcibly trying to enter the building, during which the UN said he assaulted two security officers. Donat insists he did not use any force but was simply arguing his case with the guards.

Donat, a 54-year-old US-Mauritian citizen normally based at the UN Geneva headquarters, said he had started a hunger strike on 1 March “in an entirely personal capacity” to protest about the killing of children, first by Hamas on 7 October, but since then in far greater numbers by Israeli forces in Gaza. He has worked in Gaza and said he was also protesting about the killing of more than 160 UN workers there since 7 October.

According to Donat’s account, he arrived at the UN visitor entrance on Monday and was told by a UN security official he could not protest within the headquarters and would need a New York police permit to protest outside. He says he went back to his hotel to leave his bags, his guitar and other protest paraphernalia, and then returned to the New York headquarters on Monday evening for a coffee meeting he had arranged with a UN colleague.

He said he was then denied entrance by UN security guards, one of whom told him he would be arrested by New York police if he did not leave. Donat provided video of the argument to the Guardian in which he insisted he had a right to entry as a senior member of staff and challenged the guards to carry out their threat to call the police.

Shortly after 8pm, Donat said the guards abruptly started pushing him backwards.

“At that moment, very forcefully, I think about two or three [guards] grabbed me quickly and forced me out,” he said. “I was in shock when I was pushed … from the vertical position backwards and I fell on the back of my head.”

A UN spokesperson, Farhan Haq, said that he was not aware of any ban on Donat entering the headquarters, only on protesting on the premises. But Haq said Donat was now banned from entry because, according to the guards, “he assaulted two security officers during the course of the event”.

Donat denies assault, saying he only leaned towards the guards pushing him in an effort to stay upright.

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He is currently a senior humanitarian affairs officer who, while retaining the title of head of the UN Mine Action Service in Geneva, is serving on secondment to the UN Justice and Corrections Service. Donat, however, said he was considering his future at the UN, and whether to resign.

“I’m a believer in the work of the United Nations. I have given my life to the United Nations, and I am a supporter of the current secretary general. I think he’s taken brave stances,” he said, but he added: “The UN I see now I don’t recognize from the UN that I signed up for.”

Donat said: “I am talking to friends in Gaza and Israel and the most heartbreaking message I get is my own UN colleagues telling me about the deafening silence of other UN colleagues.”