The number of unaccompanied children crossing the Mediterranean on unseaworthy boats has soared this year, the UN children's agency says.

The UNICEF said in a report on Tuesday that nine out of every 10 children arriving in Italy from North Africa were not accompanied by adults.

A total of 7,009 unaccompanied minors made the journey in the first five months of the year, "twice as many as last year.

"Italian social workers claim that both boys and girls are sexually assaulted and forced into prostitution while in Libya, and that some of the girls were pregnant when they arrived in Italy, having been raped," the document said.

Sarah Crowe, UNICEF spokesperson, said it was not clear why more minors are arriving unaccompanied.

There is concern over a sharp increase in Nigerian women and girls leaving Libya for Italy, with the International Organization for Migration (IOM) estimating 80 percent of them are victims of trafficking.

"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," 16-year-old Aimamo told UNICEF of the farm in Libya where he and his twin brother worked for two months to pay the smugglers.

INTERACTIVE: Why on earth would anyone do this journey?

Tens of thousands of children are in danger each day and hundreds of thousands more are ready to risk everything to make the journey, the agency said. 

And with the arrival of summer in Europe, the numbers of those risking the Mediterranean crossing from Africa and the Middle East are set to rise, it warned.

There are currently 235,000 refugees and migrants in Libya and about 956,000 in the Sahel countries, and "many - if not most - of them" are hoping to make their way to Europe, the UNICEF report said.

It also voiced concern over the number of the children who prefer not to register themselves upon their arrival in Europe, choosing instead to continue their journey through Europe and falling prey to criminal gangs.

"Every country - those the children leave, those they cross and those in which they seek asylum - has an obligation to establish protection systems focused on the risks that unaccompanied children face," said Marie-Pierre Poirier, UNICEF's special coordinator for Europe's migrant crisis.

Children on the move "have endured war, persecution, deprivation and terrible journeys", she said.

"Even when they have reached the relative safety of their destination, they still need protection, education, healthcare and counselling. We must be by their side."

Source: Agencies