The Islamic State group has claimed responsibility for the killing of four aid workers and a security guard in Nigeria's north-eastern Borno state.

UN Secretary General Antonio Guterres has called on the Nigerian authorities to bring the perpetrators to justice.

A photograph, published by the Islamic State's newspaper, shows five men wearing civilian clothing sitting on the ground, blindfolded.

Standing behind them are five other armed men wearing military clothing, with their faces covered.

The image appears to be a screen grab from a propaganda video. The Islamic State in West Africa Province, Iswap, is a splinter group of the Islamist extremist group Boko Haram.

The aid workers were kidnapped last month and their deaths confirmed earlier this week.

It underlines the risks humanitarian workers face in north-eastern Nigeria.

The region has seen more than a decade of violence from Boko Haram that has forced more than two million people from their homes and killed more than 30,000.