Two Russian mercenaries are believed to have been killed by ISIS after refusing to recant their faith and become Muslim.

Roman Zabolotny, 39, and Grigory Tsurkanu, 38, were thought to be fighting for a private Russian mercenary force in Syria. They appeared in a video released earlier this week by ISIS' news outlet Amak after being captured by militants. The Russian Orthodox Church has declared them to be martyrs, according to Aleteia. 

Senior Russian MP Viktor Vodolatsky said he was 99 per cent sure the men were dead.

'Before filming that video they were given a statement which they had to read,' he added according to the Mail Online.

'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.'

It was previously reported that Zabolotny was a devout Orthodox believer and went to Syria in order to protect Christians from terrorists, according to the Mail Online.

The Russian government has not confirmed their death and Vladimir Putin's regime is typically reluctant to admit the presence of mercenary soldiers in the area despite experts saying they work closely with the Moscow's forces.