The UN called on Mozambique to investigate reports that militants had massacred villagers and beheaded women and children in a restive northern region.

As many as 50 people have died in recent days in attacks by fighters linked to Islamic State, local media including Mediafax and The Pinnacle News have reported.

Violence had surged this year in Cabo Delgado - a province that borders Tanzania and is the site of a multibillion-dollar natural gas project - alarming governments across southern Africa.

UN secretary-general António Guterres called for an investigation late on Tuesday.

"The secretary-general is shocked over recent reports of massacres by non-state armed groups in several villages, including the reported beheading and kidnapping of women and children," his spokesperson said.

There was no immediate response from the Mozambican government.

Militants seeking to carve out an Islamist state started attacks in Cabo Delgado in 2017. They have seized key towns for brief periods and hit military targets this year.

In September, Amnesty International accused Mozambican soldiers of committing atrocities during a crackdown on the violence, but the defence ministry dismissed the reports, saying militants regularly impersonated soldiers.