This 16-year-old tells Mike Williams about the alleged rape by a Brazilian serviceman

The heavily armoured United Nations patrol rolls through the dusty streets of Cite Soleil - the most dangerous and deprived part of a very dangerous and deprived country.

UN peacekeepers crouch low in the turrets of the armoured cars, their rifles tracking the rooftops and alleyways. They come under fire every day in this part of the capital, Port au Prince.

The week before I arrived, two of the peacekeepers were killed there.

Exploitation

There are about 9000 peacekeepers in the UN mission to Haiti, most of them soldiers who come from 19 different nations. Most of them have come to help. They work hard in dangerous conditions to bring security and aid to the desperate people.

"He held me down by the arms and held both my wrists, twisting them back and we struggled together 16-year-old girl UN troops face abuse claims

But there are some peacekeepers who are willing to use their advantages to exploit some of the most vulnerable people in this troubled society.

I spoke to a 14-year-old girl who told of the peacekeeper who offered her jelly, sweets and a few dollars for sex with her and her friend - a child of just 11 years.

Half of the population of Haiti struggle to survive on just a dollar a day and the streets are filled with people selling whatever they can to raise a little cash. At nighttime, those who have nothing to sell, sell themselves.

Among the UN soldiers and civilians, they can find willing buyers. One UN official told me that a great many of the girls who work the streets are children and, in the dark streets of the capital Port-au-Prince, we watched UN officials picking up young prostitutes and driving off with them.

'Betrayal'

Sarah Martin, of Refugees International, has studied the problem in UN missions across the world.

Mike Williams talks to a 14-year-old who was offered food and $20 in return for sex (face blurred)

"To prey upon the very populations that you are sent to protect is one of the worst forms of violation and betrayal that there is," she says.

Sarah (not her real name) is a fragile looking girl of 16. She says that two years ago, she was raped by a Brazilian soldier serving with the UN mission there.

She stared at the ground while we talked and, almost in a whisper, she explained what happened: "He held me down by the arms and held both my wrists, twisting them back and we struggled together. And then he raped me."

Her mother cried while she recalled that day: "When I found her I didn't recognise my own child," she says. "She had the face of a dead person - I started to cry out, she couldn't tell me what had happened."

The family have been seeking justice from the United Nations but officials at the local UN mission say that justice was done. Three internal inquiries found there was insufficient evidence against the man and he was sent back to his unit in Brazil.

Immunity

Soldiers serving with the UN have immunity from local laws and it's up to their home countries to discipline them. More often than not, they're simply repatriated and the UN has little information about what, if anything, happens to them then.

All of our missions are in areas that are economically deprived... where habits like prostitution of very young children is seen as a matter of course Jane Holl Lute, assistant secretary-general for peacekeeping operations

"The UN has to be absolutely vigilant that those troops that are conducting these practices are dismissed," says Anna Jefferys of Save The Children. "It has to ensure that those member states that are deploying these troops are somehow shamed within the UN system so that the stigma becomes too big to do it again."

The UN is holding a conference in New York on Monday 4 December, at which officials will hear from victims, NGO workers and researchers in the field.

The assistant secretary-general for peacekeeping operations, Jane Holl Lute, says they need find ways to control the exploitation and she admits that the organisation has a very serious problem.

"My operating presumption that this is either an ongoing or potential problem in every single one of our missions," she says.

"All of our missions are in areas that are economically deprived, where societies have been torn by conflict and war, where habits like prostitution of very young children is seen as a matter of course.

"We need to bring every resource we can to bear to make that not the case when a peacekeeping mission is in place."

Ms Lute said the UN's inability to impose punishments was a shortcoming in the system and she admitted that the organisation does not have a system of justice that everyone would recognise as fair and equitable.

Sarah, the girl who claims she was raped by a peacekeeper would probably agree.