GENEVA - The number of unaccompanied children making the notoriously dangerous Mediterranean crossing aboard unseaworthy boats has more than doubled this year, the UN children's agency said in a new report Tuesday.

Entitled "Danger every step of the way", the UNICEF report said nine out of every 10 children arriving in Italy were unaccompanied minors, noting that more than 7,000 of them had arrived in the first five months of the year.

Unaccompanied and separated children are at particular risk of abuse and exploitation, notably by the smugglers they rely on to get to Europe, UNICEF said.

"Just about every child who arrives on the Italian island of Lampedusa or in Sicily has a harrowing story to tell," the report said.

Both boys and girls are subjected to sexual violence and forced into prostitution. Some of the girls are pregnant by the time they arrive on European shores.

"During the crossing, several people fell overboard and drowned, and others inside the boat fainted and died. I was even sitting down with dead bodies and I was scared," 17-year-old Nigerian orphan Peace said in the UNICEF report.

She had fled Nigeria to escape an arranged marriage to a 40-year-old. "I wish my friend had told me this is how difficult it is. I would have continued suffering in Nigeria," she added.

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.

Since the first of the year, 2,859 people have died while attempting to cross the Mediterranean, many of them children, according to the IOM.

The IOM figure for the whole of last year was 3,770. "The reason we are seeing more (unaccompanied children) is not clear at this stage," UNICEF's Sarah Crowe told a press conference in Geneva.

"We should never forget that children on the move are first and foremost children, who bear no responsibility for their plight, and have every right to a better life," said Marie-Pierre Poirier, UNICEF's special coordinator for Europe's migrant crisis.

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 some 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.

UNICEF also voiced concern over the number of these migrant 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.

Unaccompanied children need special protection and attention, both from the countries they leave and those where they arrive, said Poirier.

"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."

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."