Recent fighting in eastern Democratic Republic of Congo has led to a surge in child abductions by armed groups, the charity Save the Children says.
All sides in the conflict use children, Save the Children says
The situation for children in the east of the country is "catastrophic", says a report for the aid organisation.
Around the town of Goma, all sides in the conflict are making children act as soldiers, sex slaves, porters and spies, the charity said.
Conflict in the east has driven about 800,000 people from their homes.
Save the Children says it has managed to free about 800 youngsters from the militias in the last year - although some have been captured again.
Hussein Mursal, the Congolese director for the charity, said: "The situation for children in eastern DRC is catastrophic.
"Fighters from all sides are using children as frontline fodder."
Schools have been targeted by the militias as "rich recruitment ground", the charity said.
In the intensifying conflict all sides have recruited child soldiers as young as 10 years to boost their cause.
Save the Children spokeswoman Sarah Jacobs told the BBC's World Today programme about one of the child soldiers she had met around the town of Goma.
"I spent some time with a 15-year-old boy who had just escaped, so he had run for two days, no water, no food, fearing death to escape a militia group," she said.
"Listening to his tales - he had been forcibly taken; he had been fighting on the front line; his eyes were damaged from the gunpowder.
"He had been forced to kill, at point blank range, four children in his own militia group, so essentially his friends, because they were being punished; but all he talked about was wanting desperately to get back to his family."
Fighting in the area escalated at the start of this month, when the army launched a long-planned offensive against dissident general Laurent Nkunda, with the support of United Nations troops.
BBC Africa analyst Martin Plaut says the offensive went badly wrong and the general quickly gained the upper hand.
Gen Nkunda says his forces are protecting DR Congo's Tutsi population from Rwandan Hutu rebels, who have been based in eastern DR Congo since the Rwandan genocide in 1994.
He told the BBC he was not interested in using child soldiers.
"We are demobilising them, in three years I have demobilised around 2,500, so I would not be recruiting them," he said.
He also called for a ceasefire ahead of an internationally sponsored peace conference due to take place in Goma in a few days time.
"I wanted to convince our government to call for a ceasefire because we cannot undertake negotiations and talks when there is fighting," Gen Nkunda told the BBC.