The Coptic Catholic Patriarch of Alexandria has expressed gratitude to Egyptian government for its swift response against Islamic State extremists who beheaded 21 Christians they kidnapped this week.

Coptic Patriarch Ibrahim Isaac Sidrak offered condolences to the families of the victims.

He assured them that while they lost their loved ones to terrorists, they could take comfort in the arms of Egyptians rallying to express outrage over the killings.

Sidrak also called the victims, whose beheading had been captured on video and posted on the Internet, "martyrs of the faith" and pledged that their deaths would not be in vain.

In a statement, the patriarch "offers his condolence to all the families of these martyrs who gave their lives for the faith, and at the same time expresses his gratitude to President Abdel Fattah al Sisi and all the institutions of the Egyptian government for giving an immediate response to this act of terrorism."

The World Council of Churches joined its member churches, partners and people in Egypt mourning the killings.

"We express our deepest sympathy and condolences to the families of the young Egyptian men who had travelled to Libya to work for their support, and who became the victims of this outrage," said Rev. Olav Fykse Tveit, the WCC general secretary.

He wrote a letter to Pope Tawadros II, patriarch of the Coptic Orthodox Church in Egypt on 17 February.

"This tragedy is uniting the entire country, Christians and Muslims," said Father Hani Bakhoum Kiroulos, Coptic Catholic Patriarchate secretary, in an interview with Fides news agency.

"If their aim was to divide us, they have failed."

Kiroulos pointed out that among the first institutions to castigate the brutal killing of the Egyptians by the jihadists was from Cairo's Al Azhar University, considered to be an important center of Sunni Islam.

"The swift retaliation on the part of the Egyptian air force on Islamic State bases in Libya also demonstrated that for the Egyptian government its citizens are all equal and that Egypt is suffering as a nation from the bloodthirsty delirium of the terrorists," the priest continued.

On Feb. 16, the Egyptian air force bombed several areas known to be strongholds and bases of the IS in Libya, retaliating after the video of the Egyptian Christians spread online.

In a separate statement, the Egyptian government maintained it had a right to defend its people from "any criminal attacks on the part of individual terrorists or groups inside and outside the nation."

The government also announced seven days of national mourning for the victims.