A new report for the United Nations children's agency, UNICEF, has found around 120 million girls around the world have been raped or sexually assaulted by the age of 20.

In a global study of violence against children, the child welfare agency UNICEF reveals that one fifth of all murder victims are children and adolescents under age 20.

Homicide is the leading cause of death among boys and young men aged 10 to 19 in Latin American countries including Venezuela, Colombia, Panama and Brazil.

UNICEF says the report titled "Hidden in Plain Sight" is the largest-ever study of violence against children, drawing on data from 190 countries.

"These are uncomfortable facts - no government or parent will want to see them," said UNICEF Executive Director Anthony Lake.

"But unless we confront the reality each infuriating statistic represents - the life of a child whose right to a safe, protected childhood has been violated -- we will never change the mind-set that violence against children is normal and permissible. It is neither."

The report also found one in three schoolchildren aged 13 to 15 are bullied worldwide. 

As for violent discipline, the study found that about 17% of young people in 58 countries were subject to severe forms of physical punishment, including being hit on the head, ears or face or being hit hard and repeatedly.

Over 40% of children aged two to 14 years old experience severe physical punishment in Chad, Egypt and Yemen.

Globally, three in ten adults believe physical punishment is needed to raise children well.

In Swaziland, 82% say physical punishment is necessary.

Cultural attitudes towards violence

Around 126 million adolescent girls believe a husband is justified in hitting his wife under certain circumstances.

The proportion rises to 80% or more in Afghanistan, Guinea, Jordan, Mali and Timor-Leste.

In 28 of 60 countries with data on both sexes, a larger proportion of girls than boys believe that wife-beating is sometimes justified.

In Cambodia, Mongolia, Pakistan, Rwanda and Senegal, girls are around twice as likely as boys to think a husband is sometimes justified in hitting his wife.

Data from 30 countries suggest that about seven in 10 girls 15-19 years old who had been victims of physical and/or sexual abuse had never sought help: many said they did not think it was abuse or did not see it as a problem.

The UN report also tackles the mindsets it says perpetuate and justify such violence.

It recommended six strategies for preventing violence against children. 

They include "supporting parents and equipping children with life skills; changing attitudes; strengthening judicial, criminal and social systems and services; and generating evidence and awareness about violence and its human and socio-economic costs, in order to change attitudes and norms."