David Beasley, the U.N. agency's executive director, said in a recent interview with The Associated Press that he is "making hard choices to bring change" to the WFP.

"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, he told the AP in a phone interview from the agency's Rome headquarters.

The warning comes after an internal survey which Beasley commissioned found that at least 28 employees said they experienced rape or sexual assaults while working at the agency. More than 640 others said they were victims of or witnessed sexual harassment, or 8% of the total sample of 8,137. The survey was first reported last month by The Italian Insider.

The findings are lower than the U.N. Safe Space Survey, which was conducted across the world body's agencies and was published in January. Of its more than 30,000 respondents, 38.7 percent said they experienced sexual harassment while working in the U.N.

Over the past year, Beasley said that the agency fired and banned five staffers implicated in sexual abuses, doubled the number of its investigators, lifted the time limit on reporting abuses, and is spreading the message among staffers to speak up about abuses of power.

Some advocates and WFP employees question whether the agency had the ability to adequately investigate itself. The two staffers spoke on condition of anonymity because of the sensitivity of the subject.

"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. She said it was largely ignored and was never investigated.

She also said that harassers are typically forced to resign, not fired, allowing them to seek employment with other U.N. agencies. She said her agency recently hired an official accused of harassment at the World Health Organization.

A second senior official at WFP ticked off a list of names of officials at the agency's regional offices 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 U.N. employee who founded the Code Blue Campaign, which seeks to end impunity for sexual offenders in the U.N. system, called the 28 cases of rape and sexual assault "shocking and disgusting."

Donovan said the U.N. has a conflict of interest. "We are the employer of the accused and the accuser," she said, calling for an independent body from outside the U.N. to investigate.

"You can double, triple the number of investigators," she said. "But as long the results are handled internally and turned over to other biased individuals to make judgments and final decisions, none are real changes."

Sexual abuses are rife especially in remote areas away from the spotlight.

Local employees in other international agencies operating in Yemen complain that senior officials have free rein to exploit female staffers. The staffers said woman are also prevented from speaking out against sexual abuses by broader cultural and social restraints in Yemen, where women who complain of harassment often face punishment.

A senior female employee at one non-U.N. group said, "the strong majority of Yemeni women working for international agencies suffer silently from sexual harassment."

The United Nations has been in the spotlight for several years over allegations of child rape and other sexual abuses by its peacekeepers, especially those based in Central African Republic and Congo.

According to U.N. figures, there were 80 allegations of sexual exploitation and abuse involving peacekeepers and 65 allegations involving U.N. civilian staff in 2016 — an increase from 2015, according to the most recent data.

In 2017, new allegations were leveled against several agencies, including the UNHCR, the refugee agency, which helps more than 22 million people. Other allegations of misconduct involved civilians working for the U.N.'s International Office for Migration, and one with the children's agency UNICEF.

Cases of sexual harassment have also rocked other international aid agencies.

Medecins Sans Frontieres, the Switzerland-based agency also known as Doctors Without Borders, said last year it has taken action on 24 cases of sexual harassment or abuse among employees last year and dismissed 19 people.

The British-based aid agency OXFAM has also been engulfed with scandals for covering up cases of workers involved in sexual exploitation of women and girls they were supposed to help in Haiti after the 2010 earthquake.

WFP's Beasley said over the past year he has increased the number of investigators at the agency to 22 to look into allegations of misconduct, including a number who specialize in dealing with victims of sexual violence.

He said he lifted the time limit on reporting abuses and granting anonymity to the victims to encourage them to speak up. The agency has launched awareness-raising sessions for more than 3,400 staffers on "behavioral standards" to help employees identify abusive conduct and learn how to report it. He said he is also working for "gender parity," appointing more women to senior positions.

Beasley said he was working to ensure staffers removed for sexual abuse or harassment don't work at any U.N. agency. Besides two staffers fired by the WFP, three others who left the agency were banned from future employment elsewhere in the world body. He said managers who recommend those who proved to be abusers will also be questioned and could face discipline.

"I have said before and I will say it again any allegation of sexual assault will be vigorously investigated and, where substantiated, people will lose their jobs," Beasley warned in an email to WFP staffers obtained by the AP. "Not only that, I will not hesitate to take appropriate steps to refer criminal behavior to law enforcement authorities."

Asked if he is facing resistance from inside the agency, Beasley told the AP, "There are a lot of good people ... but there are also a lot of people who need to improve their management skills. It's an old culture with good and bad."

"People don't like change," he said. "We seek to be a role model to the world ... I am not happy, but we are making progress."