Two Russian mercenaries who were slaughtered by ISIS after they refused to to be filmed rejecting their Christian faith and converting to Islam have been remembered as martyrs by the  Russian Orthodox Church.

Roman Zabolotny, 39, and Grigory Tsurkanu, 38, were thought to be fighting for a private Russian mercenary force in Syria when they were captured by ISIS last week, the Mail Online reports.

The two men appeared in a video released by ISIS' news outlet, Amak, in which they are shown in a jihadist vehicle, their legs bound by rope and hands tied behind their backs. The pair were executed the same day the video was made in a town square, according to Senior Russian MP Viktor Vodolatsky.

"It is very sad but 99 per cent Roman Zabolotny is not alive, nor is the second prisoner," he said.

"Before filming that video they were given a statement which they had to read," he continued. "In this text they would reject their Orthodox religion, reject their motherland, become Muslim and join ISIS. They stayed loyal to the Orthodox faith and their Motherland until the very end, and this is what they were killed by those gangsters for."

Zabolotny was reported to have been a devout Orthodox believer who went to Syria in order to protect Christians from terrorists. Both men were believed to be fighting for a group called Wagner private army.

Ruslan Leviev, founder of Conflict Intelligence independent investigation group, said the pair were mercenaries despite the reluctance of the Russian government to acknowledge the presence of mercenary soldiers in the area.

"Our experience of watching this conflict tells us that Wagner private army mercenaries are the first to fight," he said."We think it is a strategy of the Defense Ministry of Russia: sending mercenaries to the hottest places, we avoid losses among official soldiers and keep the image of a successful combat operation."

A Russian group had offered to pay $1 million to ISIS for each of the men if they were freed. Aleteia reports that the Russian Orthodox Church this week declared them to be martyrs. 

Meanwhile, Libya's interior ministry has said they have found a mass grave with the bodies of the 21 Coptic Christians murdered by ISIS in 2015.

"The heads are separated from the bodies clad in orange jumpsuits, hands bound behind the back with plastic wire," said the ministry's unit for fighting organized crime in the city of Misurata, according to Agence France Presse.

The authorities came to know about the mass grave near the one-time Islamic State bastion of Sirte, 280 miles east of Tripoli, after an IS prisoner made a confession.

The remains, including the bodies of 20 Egyptians and a man of unknown African nationality, have been transferred to a forensics lab in Misrata city for DNA testing, according to Al-Masry Al-Youm.

The 21 Copts are being remembered as the "martyrs of Libya" and last year were officially registered in the book of martyrs by Coptic Orthodox Patriarch Tawadros II.