JOHANNESBURG (Reuters) - The United Nations 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 multi-billion dollar natural gas project - alarming governments across southern Africa.