A leading aid agency says that children as young as 11 are being beheaded in Mozambique's Cabo Delgado province, BBC reported.
One mother told Save the Children she had to watch her 12-year-old son killed in this way close to where she was hiding with her other children.
More than 2,500 people have been killed and 700,000 have fled their homes since the insurgency began in 2017.
Militants linked to the Islamic State (IS) group are behind a conflict in the province.
In its report, Save the Children did not specify who was behind the attacks, but it said that displaced people had reported gruesome scenes in the gas-rich northern province which borders Tanzania.
Chance Briggs, Save the Children's country director in Mozambique, said the reports of attacks on children "sicken us to our core."
"Our staff have been brought to tears when hearing the stories of suffering told by mothers in displacement camps," he said.
The United Nations special rapporteur on extra-judicial executions described the militants' actions as "cruel beyond words."