Witnesses say the assailants herded victims onto a football pitch in the village of Muatide where they were killed.
The United Nations has called on Mozambique to investigate reports that an armed group murdered villagers and beheaded women and children in a violence-wracked northern region.
As many as 50 people have died in recent days in attacks by fighters linked to ISIL (ISIS), local media reported.
Violence has 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 in a statement 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 spokesman said.
There was no immediate response from the government.
Security forces in Cabo Delgado have been fighting the armed group – which pledged allegiance to ISIL last year – since 2017.
Little is known about the fighters who call themselves al-Shabab – although they have no known links to the group of that name operating in Somalia.
The unrest has killed more than 2,000 people since 2017 – more than half of them civilians, according to the US-based Armed Conflict Location & Event Data Project.
The violent attacks in Cabo Delgado have triggered a humanitarian crisis with more than 300,000 internally displaced people and 712,000 in need of humanitarian assistance, according to an Amnesty International report released last month.
In September, Amnesty International accused Mozambican soldiers of committing atrocities during a crackdown on violence, but the defence ministry dismissed the reports, saying fighters regularly impersonated soldiers.