Uganda's rebel Lord's Resistance Army (LRA) has been accused of going on a massive forced recruitment campaign in remote areas of central Africa.
Human Rights Watch said the group had brutally abducted at least 697 adults and children over the past 18 months.
Civilians were said to have been taken in remote regions of the Central African Republic (CAR) and the north of the Democratic Republic of Congo.
The LRA, led by Joseph Kony, has fought the Ugandan government since 1986.
Its fighters, who are being hunted by Ugandan special forces, are now spread across northern DR Congo, Southern Sudan and the east of the CAR.
"They've been carrying out mass-scale abductions in order to replenish their ranks," Anneke Van Woudenberg, a senior Africa researcher at Human Rights Watch, told the BBC's World Today programme.
Some of those abducted managed to escape, bringing with them tales of children forced to kill other children and trained to treat other human beings as animals.
"The LRA tied the hands of the victims behind their back, a cord around their legs, and placed the victims face down on the ground," a 12-year-old Congolese girl told Human Rights Watch.
"Then the LRA would give us children a heavy wooden stick and force us to beat them on the head till they died."
The Human Rights Watch report says girls are being used for sex or as servants. Refusing sexual relations is often punished by death.
"We've conducted interviews with hundreds of people over the course of the past month. And the evidence is overwhelming that this is indeed being ordered by the LRA's leader Joseph Kony and being carried out by his top commanders," Ms Van Woudenberg said.
To escape these activities, 54,000 villagers have fled from their homes in the Bas Uele region of northern DR Congo.
Mr Kony began his rebellion 20 years ago, claiming to want to install a Bible-based theocracy in Uganda.
He is wanted by the International Criminal Court (ICC), and now lives an itinerant life, crossing between Sudan and the CAR.
In 2008, he was about to sign a peace deal, negotiated by Southern Sudan, but at the last minute he refused to lay down his arms.
In May, the US passed legislation promising a comprehensive strategy to protect civilians from LRA attacks.
Human Rights Watch is calling for swift action by the US and African governments to bring the LRA leaders to justice, in order to end their campaign of violence.
Ms Van Woudenberg said the group no longer had a political objective.
"This is no longer a group with an ideology. This is a group that survives on sheer brutality," she added.
"I really think that's why the governments of the region, but also the international community, need to end this problem. The targeting of children on such a deliberate scale, on such a large scale, is something that is a crime against humanity."