About 120 million girls around the world - slightly more than one in 10 - have been raped or sexually assaulted by the age of 20, a UN report says.

The violence "cuts across boundaries of age, geography, religion, ethnicity and income brackets,'' Unicef executive director Anthony Lake said.

"It occurs in places where children should be safe, their homes, schools and communities.

"Increasingly, it happens over the internet, and it's perpetrated by family members and teachers, neighbours and strangers and other children.''

One in three girls, aged between 15 and 19, who had at some time been in cohabiting relationships, had been victims of emotional, physical or sexual violence committed by their husbands or partners, the report said.

Of the countries surveyed, nearly half of all girls aged 15-19 believed that a husband was justified in hitting his wife under certain circumstances, the study added.