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.

The Egyptian government and the Coptic Church confirmed the authenticity of the footage, released on Sunday.

It showed the Egyptian workers, all wearing orange jump suits, being beheaded near a waterfront said to be located in the Libyan province of Tripoli.

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

Al-Azhar stresses that such barbaric action has nothing to do with any religion or human values. Al-Azhar , centre of Islamic learning

In the wake of the video release, 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 group has released several propaganda videos boasting vows of allegiance from fighters in the country. In October, Ansar al-Sharia in Derna pledged allegiance to ISIL.

Libya's embattled parliament, which is locked in a conflict with militias, expressed its condolences in a statement and called on the world to "show solidarity with Libya" against ISIL.

The UN's mission in Libya called for the group's actions to be "rejected and denounced by all Libyans".

A scrolling caption in the video referred to the hostages as "People of the cross, followers of the hostile Egyptian Church".

Speaking in English, a fighter from the group said the beheadings were revenge for "Muslim women persecuted by Coptic crusaders in Egypt".

Sunday's video comes less than two weeks after ISIL released a video showing the burning alive of a Jordanian pilot it captured after his plane went down in Syria in December.