The M23 group, which began with a military mutiny, has killed at least 33 boys and young men who tried to leave, Human Rights Watch said.
The Rwandan military has supported M23 operations, the group said.
"The M23 rebels are committing a horrific trail of new atrocities in eastern Congo," said Anneke Van Woudenberg, senior Africa researcher at Human Rights Watch. "M23 commanders should be held accountable for these crimes, and the Rwandan officials supporting these abusive commanders could face justice for aiding and abetting the crimes."
Human Rights Watch says M23 is responsible for at least 46 rapes, including one where the victim was only 8 years old. But M23 leaders deny atrocities or the forced recruitment of child soldiers, blaming it on DRC government propaganda.
"We recruit our brothers, not by force, but because they want to help their big brothers. ... That's their decision," a leader identified as Col. Makenga said. "They are our little brothers, so we can't kill them."