“It’s a brutal, terrible reality, but young people need to know the risks before deciding to go,” said Christopher Tidey, a Unicef spokesman. “Bottom line: This proves how essential it is that migrant and refugee children have access to safe and legal migration pathways.”

The report is one of the first attempts to use both anecdotes and quantitative research to document the abuse of migrants based on a variety of factors, including country of origin. It analyzes the testimony of some 22,000 migrants and refugees trying to cross the Mediterranean, focusing on those who are age 14 to 24.

The report offers an example: An adolescent boy from sub-Saharan Africa, even one who has secondary education and travels in a group along the Central Mediterranean route, faces a 75 percent risk of being exploited. If he came from another region, where skin tones are lighter, the risk would drop to 38 percent, it says.

“Countless testimonies from young migrants and refugees from sub-Saharan Africa make clear that they are treated more harshly and targeted for exploitation because of the color of their skin,” the report says.

Tensions between North Africans and sub-Saharan Africans have long existed.

In numerous sub-Saharan African countries, unemployment is soaring, prompting young men and women to leave home to get to Europe, where they hope to find work. Recent statistics have shown that the migrant flow to Europe has slowed, but no one is certain why. Analysts have cautioned that the lull is unlikely to be permanent.

The report says that while young people are at greater risk than adults regardless of where they cross the Mediterranean, those who cross from Libya are in the greatest danger. In Libya, “they contend with pervasive lawlessness and violence and are often detained, by state authorities and others,” it says.