Fighters pledging allegiance to the Islamic State of Iraq and the Levant (ISIL) have released a video purporting to show the killing of 21 Egyptian Coptic Christians kidnapped in Libya.

In the video, ISIL referred to the hostages as “People of the cross, followers of the hostile Egyptian Church”. A fighter from the group said in English that the beheadings were revenge for “Muslim women persecuted by Coptic crusaders in Egypt”.

The men were seized in two attacks in December and January from the coastal town of Sirte in eastern Libya.

Egyptian President Abdel Fattah el-Sisi called for an urgent meeting of Egypt’s top national security team and declared seven days of mourning.

“Egypt reserves the right to respond in a suitable way and time to punish these murderers,” Sisi said in a televised speech.

Later, state television reported that Egypt’s military had bombed ISIL targets in Libya at dawn on Monday.

The Coptic Orthodox Church issued a statement saying it was “confident” the killers would be brought to justice.

Al-Azhar, the prestigious Cairo-based seat of Islamic learning, denounced the “barbaric” killings.

“Al-Azhar stresses that such barbaric action has nothing to do with any religion or human values,” it said in a statement.

Libya has slid into chaos after longtime leader Muammar Gaddafi was overthrown and killed three years ago, as interim authorities failed to confront powerful militias which fought to oust the authoritarian leader.

Taking advantage of the chaos, ISIL has carried out a string of deadly attacks.

The 21 men have been declared martyrs by the Coptic Church.