Millions of refugees have fled conflicts in the region
The scale of the problem - revealed in an overview of a report by the UNHCR in conjunction with the British-based charity Save the Children - has surprised relief personnel.
An unspecified number of interviewees complained that they or their children had to have sex in order to get food and favours.
Over 40 aid agencies - including the UNHCR itself - were implicated, and 67 individuals - mostly local staff - named by the children.
Some under-age girls said United Nations peacekeepers in the West African region were involved.
Most of those who said they were abused were girls under the age of 18, but the mission said it had heard from some who were much younger.
Poverty
The report said that the practices were rife in locations with established programmes, including refugee camps in Guinea and Liberia.
But it said that poverty was the principle cause, with parents feeling compelled to offer their children to aid workers for sex in order to survive.
"They want us to love to them (sic) so they can give us money," one refugee told the BBC.
Another said she was continually sent to the back of the queue for aid because she had refused to have sex with one aid worker.
Sex could buy items of food, as well as such things as loans and scholarships.
Condoms were said to be rarely used by the staff involved.
New measures
Save the Children says it was determined to use the information obtained in the study to put a stop to the practices.
"The very people who are meant to be providing services are the exploiters themselves," said Save the Children Liberia country director Jane Gibril.
"We are going to sit down and look at what we do and how we do it and try and do things very differently... to ensure children do not have to sell sex for services," she added.
The charity has already sacked three employees.
The UNHCR has drawn up a package of remedial measures, including increasing security and the international presence in camps and the deployment of more female staff.
Other steps include establishing a mechanism to give refugees a secure channel for raising complaints with senior UNHCR staff.
But a spokesman for the agency, Ron Redmond, said the accusations were based only on testimonies from individuals and were so far unsubstantiated.
One senior aid worker said ending the sexual exploitation of under age refugees would be an uphill task because gender discrimination was deeply rooted in many cultures, not only here in West Africa but all over the world.