The head of the UN’s World Food Programme (WFP) has vowed to take “aggressive” action against sex abusers, following an internal survey that found multiple allegations of rape and sexual assault at the agency.

“If we have a claim of rape by anyone in the WFP, if we can substantiate, I can’t begin to tell you how aggressive” actions will be, executive director David Beasley told news agency Associated Press.

At least 28 employees said they had experienced rape, attempted rape or sexual assault while working at WFP.

More than 640 other people, or 8% of the 8,127 employees surveyed, said they were victims of, or had witnessed, sexual harassment. And 12%, or 950 staff, said they witnessed retaliation for speaking up about abusive practices.

The confidential draft survey, first published by the Italian Insider, revealed claims that senior management at the agency had abused their authority, committed or enabled harassment, discriminated against female employees and ethnic minorities and retaliated against those who spoke up in protest.

The claims were a blow to Beasley, who in January 2018, wrote to staff with changes he said would “bolster our zero tolerance stance” to better protect victims of sexual abuse as well as finding an punishing abusers.

A UN-wide report of 30,000 staff published in January, showed one in three employees and contractors experienced sexual harassment in the last two years. Beasley said that, over the last year, the agency fired and banned five staff implicated in sexual abuses, doubled the number of its investigators, extended the time limit for reporting abuses, and encouraged staff to speak up about abuses of power.

The WFP chief, who took over in 2017, ordered changes in the agency’s policies against harassment and abuse of power last year. The Guardian has reported sexual misconduct at several UN agencies, including the WFP.

Executive director David Beasley said the organisation has a ‘zero tolerance stance’ on harassment. Photograph: Denis Balibouse/Reuters

Some advocates and WFP employees questioned whether the agency could adequately investigate itself.

“They love PR, but in reality nothing changes,” said one female employee who filed a sexual harassment report in one of the agency’s regional offices this year. Speaking to AP on condition of anonymity, she said her report was largely ignored and never investigated.

Harassers are typically forced to resign, she said, allowing them to seek employment with other UN agencies. Her agency recently hired an official accused of harassment at the World Health Organisation, she said.

A second senior official at WFP named a number of officials at the agency’s regional offices who he said have been accused of sexual abuses but remained in their positions because of they were protected by managers.

“When managers are powerful, they become like mafias,” he said.

Paula Donovan, a former UN employee who founded the Code Blue Campaign, which seeks to end impunity for sexual offenders in the UN system, called the 28 cases of rape and sexual assault “shocking and disgusting”.

The UN has a conflict of interest, according to Donovan. “We are the employer of the accused and the accuser,” she said, calling for an independent body from outside the UN to investigate.