Children make up the majority of victims of sexual violence in many conflict and post-conflict zones and are suffering abuse and rape at an "appalling rate", a charity said.
A report from Save The Children, released ahead of a G8 meeting in London to tackle the issue, has gathered figures from a range of countries affected by conflict over the past decade, including Democratic Republic of Congo, Liberia and Colombia.
The research shows more than half of sexual violence victims were children.
Among testimonies gathered from victims and witnesses, the charity heard of children being killed after being raped, both girls and boys kidnapped and abused by armed forces and groups, and children as young as two being attacked by sexual predators, including teachers, religious leaders, peacekeepers and family members.
One example is how a study in Liberia - 10 years on from a vicious civil war - found that more than 80% of victims of gender-based violence in 2011-12 were younger than 17, and almost all of them were raped. In post-conflict Sierra Leone, more than 70% of the sexual violence cases seen by the International Rescue Committee were girls under 18, and more than a fifth of those were girls under the age of 11.
In Democratic Republic of Congo nearly two-thirds of sexual violence cases recorded by the UN in 2008 involved children, mostly adolescent girls. In Colombia, more than half of the victims of sexual violence helped by the International Committee of the Red Cross in 2009 were children. Almost one fifth of girls in Haiti's capital, Port-au-Prince, were raped during an armed rebellion in 2004 and 2005.
Despite this, programmes to prevent children falling victim to sexual violence and help them recover from attacks remain chronically underfunded, the charity said. The most recent complete global figures show that less than a quarter of the budget needed to protect children and women in emergencies was available.
Justin Forsyth, Save the Children's chief executive, said: "It is shocking that in conflict zones around the world, children are being raped and abused at such an appalling rate. Sexual violence is one of the hidden horrors of war and the damage it wreaks ruins lives. Even if they recover from the physical effects of their experiences, many victims carry the psychological scars of their ordeal for the rest of their lives, and are often cast out from society. Despite all this, there are huge gaps in funding for the work needed to protect children from these atrocious crimes and to respond to their needs."
Hollywood actress Freida Pinto, global ambassador for Plan International's Because I am a Girl campaign, went on a visit with Plan to Sierra Leone and met girls "living with the nightmare reality of sexual violence". She said: "They face tremendous hurdles - child marriage, teenage pregnancies and forced or self-imposed prostitution to make ends meet. Many of these girls were being denied an education and their chance to fulfil their potential."
The Slumdog Millionaire star said she found the visit "inspirational" and praised the girls she met for their courage. "Time and time again, these young women told me about their dreams - that they want to be accountants, doctors, journalists, politicians - even actors. These girls do not see themselves as victims - but they definitely want and need our help to achieve their full potential," she said.