The bodies of 34 Ethiopian Christians executed in Libya by Islamic State (IS) in 2015 have been exhumed from a mass grave, Libyan authorities said on Monday.

The grave was unearthed on Sunday, after information was obtained during investigations of arrested IS members, the interior ministry's Criminal Investigation Department (CID) said on its Facebook page.

A video posted on social media in April 2015 appeared to show IS militants shooting and beheading the Christians, who were wearing orange jumpsuits, on a beach.

The video, in which militants call Christians crusaders who are out to kill Muslims, showed about 15 men being beheaded on a beach and another group of the same size shot in their heads in scrubland.

Both groups of men are referred to in a subtitle as 'worshippers of the cross belonging to the hostile Ethiopian church'.

The CID displayed drone images of the mass grave, which appeared to be on a farm near the coastal city of Sirte.

The bodies will be repatriated to Ethiopia once 'domestic and international legal procedures are completed', it said.

Hundreds of thousands of people have migrated from sub-Saharan Africa to Libya in recent years, many hoping to eventually reach Europe.

Islamic State took control of Sirte, the hometown of Libya's former leader Muammar Qaddafi, in 2015 and lost the city late in 2016 to local forces backed by US air strikes.

Also in 2015, 21 Egyptian Coptic Christians were executed by ISIS in Libya. Their bodies have been repatriated to their home province of Minya and they are regarded as martyrs.