Most young refugees travelling alone, says Unicef

"What we have really been alarmed by, I would say is the enormous number of unaccompanied children and adolescents who are making this journey," said an Unicef spokesperson.

Unicef classified roughly 35 percent of the people crossing the Mediterranean from North Africa this year as adolescents or children. Of those, the vast majority are alone.

Youngsters from other African states who make it as far as Libya have often endured intense hardship along the way with many at risk of "detention, rape, forced labour, beatings or death".

The report notes some 235,000 refugees and migrants are currently in Libya with nearly another million in Sahel countries. Most want to reach Europe.

“If you try to run they shoot you and you die. If you stop working, they beat you. It was just like the slave trade,” said one 16-year-old from The Gambia who worked on a farm in Libya.

Around 16,500 were spotted heading to Libya from Niger in the last week of May.

Many who make the journey are forced into a "pay as you go" system. Some will work for a few days or even months to pay off smugglers.

Others fall prey to sexual exploitation. Among the biggest group abused for sex are women and girls from Nigeria. Around 80 percent are trafficked to Italy.

Safety is not guaranteed once in Europe. The EU's police agency Europol earlier this year estimated 10,000 refugee children have gone missing. Some are likely to have been taken by criminal gangs.

The EU commission last month in a separate report noted that at least one in four asylum seekers in Europe is a child.