UNICEF says most of the children migrants who have crossed the Mediterranean Sea from Libya to Italy this year were unaccompanied by adults, making them vulnerable to abuse and exploitation.

The children's agency says 92 percent of some 7,600 children who made the dangerous and often deadly crossing between January and May were unaccompanied minors, up from 68 percent in the same span last year, when 4,566 children crossed.

Spokeswoman Sarah Crowe told reporters Tuesday in Geneva that most were boys aged 15 to 17, and came from Somalia, Nigeria and Eritrea. She said boys and girls have faced prostitution, pedophilia rings and gangsterism. She said it's unclear why more minors are arriving unaccompanied.

Migration watcher IOM says that as of Sunday, 210,000 migrants had crossed the Mediterranean this year.