Updated: Mar 19
Children as young as 11 are being beheaded in Mozambique, Save the Children has said.
More than 2,500 people have been killed and 700,000 have fled their homes since the insurgency began in 2017.
The UK-based charity said it had spoken to families who have described "horrifying scenes" of murder, including mothers whose young sons were killed.
The rebels locally known as al-Shabab,which means The Youth in Arabic, have declared allegiance to ISIL. They are separate from a Somali armed group with a similar name.
Some analysts believe the insurgency's roots lay in socio-economic grievances, with many locals complaining that they have benefited little from the province's ruby and gas industries.Cabo Delgado is one of the poorest provinces in Mozambique, with high rates of illiteracy and unemployment.
The United States last week declared the Mozambique group a foreign “terrorist” organisation over its links to ISIL.
The first attack in Mozambique claimed by IS was in Cabo Delgado in June 2019.
Throughout 2020, the insurgents repeatedly engaged the military to capture and hold key towns.
While beheadings have always been a hallmark of the attacks, the brutality and mass killings have worsened, with the murder of around 52 people at once in the village of Xitaxi in April last year among them.
The US embassy in Mozambique on Monday said US special forces will train Mozambican marines for two months. The US government will also provide medical and communications equipment to help Mozambique combat the fighters.
The European Union also announced last year that it would provide training to Mozambican forces.
Amnesty International said earlier in March that all sides in the Mozambique conflict were committing war crimes, with government forces also responsible for abuses against civilians - a charge the government has denied.
The insurgency comes in the wake of a COVID pandemic that has hit the country hard and a devastating cyclone, in 2019, that rendered tens of thousands homeless.