LONDON, England (CNN) -- Humanitarian aid workers and United Nation peacekeepers are sexually abusing small children in several war-ravaged and food-poor countries, a leading European charity has said.

Children like this 15-year-old girl have suffered abuse at the hands of some UN soldiers and aid workers.

Children as young as 6 have been forced to have sex with aid workers and peacekeepers in return for food and money, Save the Children UK said in a report released Tuesday.

After interviewing hundreds of children, the charity said it found instances of rape, child prostitution, pornography, indecent sexual assault and trafficking of children for sex.

"It is hard to imagine a more grotesque abuse of authority or flagrant violation of children's rights," said Jasmine Whitbread, chief executive of Save the Children UK. Watch a report on the abuse »

In the report, "No One To Turn To" a 15-year-old girl from Haiti told researchers: "My friends and I were walking by the National Palace one evening when we encountered a couple of humanitarian men. The men called us over and showed us their penises.

"They offered us 100 Haitian gourdes ($2.80) and some chocolate if we would suck them. I said, 'No,' but some of the girls did it and got the money."

Save the Children says that almost as shocking as the abuse itself is the "chronic under-reporting" of the abuses. It believes that thousands more children around the world could be suffering in silence.

According to the charity, children told researchers they were too frightened to report the abuse, fearful that the abuser would come back to hurt them and that they would stop receiving aid from agencies, or even be punished by their family or community.

"People don't report it because they are worried that the agency will stop working here, and we need them," a teenage boy in southern Sudan told Save the Children.

The charity's research was centered on Ivory Coast, southern Sudan and Haiti, but Save the Children said the perpetrators of sexual abuse of children could be found in every type of humanitarian organization at all levels.

Save the Children is calling for a global watchdog to tackle the problem and said it was working with the U.N. to establish local mechanisms that will allow victims to easily report abuse.

"We are glad that Save the Children continues to shed a light on this problem. It actually follows up on a report that we did in 2002 with Save the Children. I think every population in the world has to confront this problem of exploitation and abuse of children," said Ron Redmond, chief spokesman for the United Nations High Commissioner for Refugees in Geneva, Switzerland.

"The United Nations has a zero-tolerance policy. It's one that UNHCR takes very, very seriously. In refugee camps, we have implemented very strong reporting mechanisms so that refugees can come forward to report any abuses or alleged abuses."

In 2003, U.N. Nepalese troops were accused of sexual abuse while serving in the Democratic Republic of Congo. Six soldiers were jailed.

A year later, two U.N. peacekeepers were repatriated after being accused of abuse in Burundi, and U.N. troops were accused of rape and sexual abuse in Sudan.

Last year, the U.N. launched an investigation into sexual abuse claims in Ivory Coast.

The vast majority of aid workers were not involved in any form of abuse or exploitation but in "life-saving essential humanitarian work," Save the Children's Whitbread said.

But humanitarian and peacekeeping agencies working in emergency situations "must own up to the fact that they are vulnerable to this problem and tackle it head on," she said.

The aid agency said it had fired three workers for breaching its codes and called on others to do the same. The three men were dismissed in the past year for having had sex with girls aged 17, which the charity said is not illegal but is cause for loss of employment.

Other UK charities said they supported Save the Children's call for a global watchdog.

"Oxfam takes a zero-tolerance approach to sexual misconduct by its aid workers. All our staff across the world are held accountable by a robust code of conduct," said Jane Cocking, Oxfam charity's humanitarian director.

"We support Save the Children's calls for a global watchdog. We will do all we can to stamp out this intolerable abuse."

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