The United Nations has been grappling with so many sexual abuse allegations involving its peacekeepers that Secretary-General Ban Ki Moon recently called them "a cancer in our system."

Now, officials have learned about what appears to be a fresh scandal. Investigators discovered this month that at least four U.N. peacekeepers in the Central African Republic allegedly paid young girls as little as 50 cents in exchange for sex.

The case is the latest to plague the U.N. mission in the Central African Republic, whose employees have been accused of 22 other incidents of alleged sexual abuse or sexual exploitation in the past 14 months. The most recent accusations come in the wake of Ban's efforts to implement a "zero tolerance" policy for such offenses.

As the United Nations maintains nine peacekeeping operations in Africa, employing over 100,000 people on the continent, the abuses threaten to erode the organization's legitimacy. Other sex-crime cases have occurred in Mali, South Sudan, Liberia and Democratic Republic of Congo in recent years.

Last month, the United Nations published a damning independent investigation that said poor enforcement of policies meant to deter and report abuse meant that "the credibility of the U.N. and peacekeeping operations are in jeopardy." Experts and officials say systemic problems still hinder the investigation and prosecution of alleged abusers, leading to the perception of impunity within U.N. ranks.

The abuse "undermines everything we stand for," said Anthony Banbury, the U.N. assistant secretary-general for field support.

The mission in the Central African Republic, where U.N. troops and civilians were sent in 2014 to help end a civil war and support a fledgling government, stands out for its record of sexual abuse and exploitation.

"They are preying on the people they've come to protect," said Parfait Onanga-Anyanga, the top U.N. official in the country.

The most recent allegations involve at least four peacekeepers who are accused of paying girls as young as 13 for sex at a camp for the internally displaced next to the international airport in Bangui, the capital.

The site, known as M'Poko camp, is home to 20,000 people, mostly Christians. It is a vast agglomeration of white tents surrounding old, decaying airplanes, just yards from the airport runway.

The United Nations has not publicly released the nationalities of the accused troops, or provided details of the alleged abuse. But in interviews, U.N. officials said the peacekeepers were from Gabon, Morocco, Burundi and France. The prostitution ring they allegedly used was run by boys and young men who offered up girls "for anywhere from 50 cents to three dollars," according to one official, who like others spoke on the condition of anonymity because they were not authorized to discuss the ongoing investigation.

Some officials say that there may be many more cases of exploitation by peacekeepers that have gone unreported. Because there is no regular U.N. presence in M'Poko, it has been difficult to gauge the scale of the problem.

M'Poko had already had a problem with sexual abuse before the recent cases were reported. Its population had grown sharply since September, when violence erupted between the warring parties in the Central African Republic.

Human Rights Watch documented nine cases of sexual violence between September and December in and around the displacement camp. In several instances, Christian women were raped by members of the mostly Christian "anti-balaka" militia after being accused of interacting with Muslims. Across Bangui, the capital, the conflict has fallen largely along religious lines.

"M'Poko is a lawless zone run by anti-balaka thugs a few hundred meters away from the international airport. The camp is not being protected, and women are being raped," said Lewis Mudge, Human Rights Watch's Central Africa Republic researcher.

But this marks the first time that the United Nations has acknowledged the involvement of its own employees in the camp's underworld of commercial sex work, which is driven by abject poverty and a lack of law enforcement.

"The M'Poko camp is unfortunately a place where horrible, unacceptable things happen to women and children," said Banbury. "In some cases we have credible allegations that there are U.N. personnel that have committed these crimes."

Banbury said U.N. troops plan to begin patrolling M'Poko more frequently and will attempt to dismantle the prostitution ring.

The U.N. mission in the Central African Republic has been plagued by sexual abuse allegations. The previous U.N. special representative there, retired Senegalese general Babacar Gaye, was fired in August over his team's handling of the allegations. The organization has dispatched special investigators to Bangui to better understand what has gone wrong.

The U.N. mission was also strongly criticized for failing to react to offenses by other peacekeepers in the Central African Republic. As many as 14 troops from France, Chad and Equatorial Guinea allegedly raped and sodomized six boys between the ages of 9 and 15 in 2013 and 2014, before the U.N. mission formally began. The United Nations took no action after learning about the cases, until a whistleblower leaked an internal U.N. investigation to French authorities, according to U.N. officials. Last month, the report by a panel including former Canadian supreme court justice Marie Deschamps found that U.N. staff in Bangui had "turned a blind eye to the criminal actions of individual troops" in that case.

In August, two women and one girl accused three U.N. peacekeepers of rape in the war-torn town of Bambari. That same month, a U.N. police officer allegedly raped a 12-year-old girl during an operation in Bangui's main Muslim neighborhood. She had been hiding in a bathroom while peacekeepers searched her house, according to Amnesty International.

"When I cried, he slapped me hard and put his hand over my mouth," the girl told an Amnesty International researcher.

For years, the United Nations has been trying to stop the sexual abuse perpetrated by its own employees and troops. It has ordered a series of reports to identify weaknesses in enforcement and mandated a "Sexual Exploitation and Abuse" component be included in training for peacekeepers. Ban has encouraged harsher penalties for the peacekeeping units to which the abusers belong.

But the slow pace of investigations into abuse has "severely undermined enforcement," according to a report last year from the United Nations' office of Internal Oversight Services. Even more problematic, some experts say, is that the prosecution of alleged offenders falls to the governments of the countries that provide the peacekeepers. In many cases, those governments conduct halfhearted investigations and fail to convict offenders.

"To say that it is immensely frustrating is a tremendous understatement," said Banbury.

"The U.N. should stop tiptoeing around, trying not to offend governments and instead put the victims of sexual exploitation and abuse at the heart of their policy," said Sarah Taylor, advocate in the women's rights division at Human Rights Watch.

Some advocates argue that the lack of enforcement encourages a sense among U.N. employees that they can commit sexual crimes with impunity while based overseas.

"They think 'We're in a special class,' that sexual abuse is not that serious,," said Paula Donovan, who leads Code Blue, an advocacy campaign working to expose the issue of sexual abuse by U.N. personnel.

The number of alleged cases of sexual exploitation and abuse committed by U.N. personnel has declined from 2008 to 2014, dropping from 83 to 51, which U.N. officials say is evidence of increasingly effective intervention. But critics say those numbers are incomplete, and that many cases go unreported.

"The data is not just porous. It's a joke," Donovan said.

Other analysts say that getting civilians to report sexual crimes in war-torn environments, where there is a mistrust of authority and a lack of law enforcement, is an enormous challenge. The victims might "fear retaliation by the perpetrator, who in some cases carries a weapon," said a report last year on U.N. abuses by Civilians in Conflict, a Washington-based research organization.

In many other cases, impoverished girls and women accept food and money in exchange for sex.

"This is already a society whose social fabric has totally collapsed, with youngsters left to fend for themselves," said Onanga-Anyanga. "This is putting salt into an open wound."