Girls have been part of government militia or opposition fighting forces in more than 50 countries over recent years, a Canadian human rights organisation has said in a new report.
The organisation, Rights and Democracy, said many of the girls had taken part in armed conflict, were abducted or had to join to survive.
Sexual abuse was widespread.
Colombian rebels rely on girls for about a quarter of their forces
The report, called Where are the Girls, focuses on northern Uganda, Sierra Leone and Mozambique.
According to the report, girls - defined as those under 18 - have recently been involved in armed conflicts from Angola to Sri Lanka, Colombia to Uganda.
The report says that as well as being used as porters, domestic labourers and captive wives, many of the girls became front-line fighters.
In Sierra Leone the so-called "wives" of commanders had considerable influence in the rebel compounds, sometimes organising raids, abductions and spying missions.
In northern Uganda, the opposition Lord's Resistance Army is largely made up of abducted children - a third of them girls.
Children in such rebel forces receive no pay and are sometimes sent to fight with sticks or without ammunition.
Girls are often expected to fight even when pregnant or if they have small children.
Diane Mazarana, one of the report's authors, said girls were the backbone of many rebel forces.
"In part they're going after girls because they're using their labour," she said.
"In a lot of ways we can think about these children as war slaves. They are the porters, they're the cooks, they're the frontline fighters.
"They're taking care of basically running the compounds and when the fighting starts, it's often the girls and boys that are at the absolute frontlines," she told the BBC.
The girls are often subjected to sexual violence.
Ms Mazarana said in the three countries they studied, virtually all the abducted girls had been raped.
"In particular with girls you'll be dealing with issues of sexual abuse," she said.
"Our work found that the vast majority were sexually assaulted. You'd be dealing with very high rates of sexually transmitted diseases, about 30% of the girls in the three countries we worked in became pregnant during captivity in the fighting forces and are now returning as girl mothers," she said.
The report found relatively few girls went through any disarmament or demobilisation programmes.
It said many were stigmatised because they were raped or had a baby and did not receive any help reintegrating into their communities.
The organisation said the overwhelming need for these girls once they were back home was for education and skills training.