A "hidden army" of girls is being forced to work for armed groups involved in conflicts worldwide, a charity says.
Girl guerrilla fighter in Colombia
A Save The Children report found more than 120,000 girls and young women have been abducted and pushed into conflict.
The charity says they often end up serving as soldiers or performing domestic jobs, but almost all of them are forced into becoming sex slaves.
It urged world leaders to do more to stop "the war on children".
Figures released by Save The Children showed that around 6,500 girls have been captured by the rebel Lord's Resistance Army in Uganda.
In the Democratic Republic of Congo another 12,000 are believed to be involved in armed organisations while another 21,500 are thought to be associated with the conflict in Sri Lanka (43% of all children fighting in that war).
A spokesman for the organisation said the study was not unique to any one country or continent and it showed a worrying global trend.
'Stop war on children'
The report also criticises world leaders and donors for failing to address the problem and says rehabilitation programmes set up by the international community are considerably under-funded.
It says that in post-war "disarmament, demobilisation and reintegration" (DDR) programmes, weapons collection often takes priority over rehabilitation.
Often girls get only some food, water, oil, plastic sheeting for shelter and a lift home, the report says.
"Girls returning home are often marginalised and excluded from their communities," it says. "They are viewed as violent, unruly, dirty, or as promiscuous troublemakers.
"Girls returning pregnant or with babies face the additional pressure of protecting and providing for their babies with little or no support from a community that resents their presence.
"With no other means of supporting themselves, many are forced to turn to sex work, making them even more stigmatised and isolated."
Save the Children told the story of Hawa, a 16-year-old girl, who was twice captured when her village in Sierra Leone was overrun during the war, and forced into sex slavery.
"It was very sad when I came home and met my sisters because I felt that I was somehow discriminated against because I had been raped," she said.
Director general of Save the Children Mike Aaronson said: "When people picture conflict they think of men in bloody combat, but it's horrifyingly girls who are the hidden face of war.
"This appalling abuse of girls' rights demands urgent action. Its time to stop the war on children."