Depression is the top global cause of illness and disability for adolescents, with suicide the third-biggest cause of death, the World Health Organisation (WHO) said on Wednesday.

The finding is in a new report by the UN agency, which has pulled together a wealth of published evidence with direct consultations with 10 to 19-year-olds around the world to assess the health issues that affect them.

Half of all people who develop mental disorders have their first symptoms by the age of 14, according to the World Health Organisation report, which advocates a stronger focus on the mental health of adolescents.

"The world has not paid enough attention to the health of adolescents," says Flavia Bustreo, head of the WHO's family, women and children's health division.

Some studies show that half of all people who develop mental disorders have their first symptoms by the age of 14, said the report.