UNICEF says at least 129 children were killed during a government offensive against rebel forces in South Sudan last month.
UNICEF's executive director, Anthony Lake, said survivors reported that boys were castrated and left to bleed to death while girls as young as 8 were gang raped and killed.
He said other children were reportedly thrown into burning buildings.
"In the name of humanity and common decency, this violence against the innocent must stop," Lake said.
South Sudan has been fighting rebels led by the former deputy president since December 2013, when a split among the armed forces in the capital, Juba, later escalated in bloody violence across the country.
UNICEF estimates some 13,000 children have been forced to participate in the conflict.
“Imagine the psychological and physical effects on these children – not only of the violence inflicted on them but also the violence they are forced to inflict on others,” Lake said.
Watchdog groups have accused both sides of carrying out serious rights abuses against civilians.
