Save the Children says Islamist militants are beheading children as young as 11 in Mozambique’s northern province of Cabo Delgado.
One mother told the agency she had to watch helplessly as her 12-year-old son was 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 an Islamist insurgency began in 2017.
The militants are linked to the Islamic State (IS) group.
In its report, Save the Children said it had spoken to displaced families who reported gruesome scenes in the gas-rich province.
One mother, named Elsa, told how her eldest child was beheaded near where she and her other children were hiding.
“That night our village was attacked and houses were burned,” she said.
“When it all started, I was at home with my four children. We tried to escape to the woods but they took my eldest son and beheaded him. We couldn’t do anything because we would be killed too.”
Another mother, Amelia, said her son was killed by militants while she and her other three children were forced to flee.
“After my 11-year-old son was killed, we understood that it was no longer safe to stay in my village,” she said.
“We fled to my father’s house in another village, but a few days later the attacks started there too.”
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. “This violence has to stop, and displaced families need to be supported as they find their bearings and recover from the trauma.”
Reports of Islamist militants beheading villagers in the region have emerged before.
Last November, state media reported that more than 50 people had been beheaded at a football ground in Cabo Delgado.
In April last year, dozens more were beheaded or shot dead in an attack on a village.
Human rights groups say security forces have also carried human rights abuses, including arbitrary arrests, torture, and killings, during operations against the jihadists.
Mozambique’s government has appealed for international help to quell the insurgency.
On Monday, US embassy officials in the capital Maputo said American military personnel would spend two months training soldiers in Mozambique, as well as providing “medical and communications equipment”.
“Civil protection, human rights, and community involvement are central to US co-operation and are critical to effectively combating Islamic State in Mozambique,” an embassy statement said.