"The Orthodox church... is confident its homeland would not rest until the evil perpetrators get their fair retribution for their wicked crime," the Coptic Church said in a statement on its Facebook page, AFP reports.
The ISIS terrorists group released a video earlier on Sunday purportedly showing the beheading of Egyptian Copts captured in Libya.
In the video, militants in black marched the captives, dressed in orange jump suits, to a beach. They were forced down onto their knees, then beheaded. The video appeared on the Twitter feed of a website that supports ISIS.
A caption on the five-minute video read: "The people of the cross, followers of the hostile Egyptian church."
Thousands of Egyptians have travelled to neighbouring Libya in search of jobs since an uprising at home in 2011, despite advice from their government not to go to one of the most dangerous countries in the region.
Before the killings, one of the militants stood with a knife in his hand and said: "Safety for you crusaders is something you can only wish for."