The beheading of 21 Egyptian Coptic Christians has energised the Church and strengthened relationships with the Muslim community, a Coptic-Catholic Bishop has said.

"Persecution is part of the life of the Church. The blood of martyrs is the seed of the Church," Bishop Youssef Aboul-Kheir of Sohag told Aid to the Church in Need this week.

"In Europe the Church is free. We, on the other hand, are faced with many obstacles. But which Church is the stronger?"

Islamic State issued a video on February 15 showing the beheading of a group of Egyptian Christians who had been kidnapped in Libya.

The five-minute video was titled: "A message signed with blood to the nation of the cross" and the men identified as "people of the cross, the followers of the hostile Egyptian Church." They are shown being marched to a beach and forced to kneel on the sand.

In a message to the "crusaders", a masked insurgent holding a knife says his fellow militants are "chopping off the heads of those that have been carrying the cross illusion." All 21 are then murdered.

Earlier this week the brother of two of those killed has thanked IS for including the men's declaration of faith in the video.

The last words of some of those killed were "Lord Jesus Christ" and Beshir Kamel said the men's show of faith has strengthened his own.

Bishop Aboul-Kheir has also praised the dedication of those killed. "They suffered a holy death with prayers on their lips. They went to their deaths just like the early Christians," he said.

And despite attempts to disrupt Muslim-Christian relations in Egypt, IS' brutality has in fact done the opposite, he said. "Many Muslims are angry because of the murders. President Sisi visited the leader of the Coptic Church to convey his condolences. And the President travelled to the home of the murdered ones. You can see that the attack has united us Egyptians," the bishop explained.

However, he expressed concern about Muslim extremists who "speak with forked tongues".

"It is crucial that we Christians in Egypt should finally be able to live as equal citizens," he said.