WARNING: GRAPHIC CONTENT. Islamic State has released a video showing 21 Egyptian Christians being beheaded in Libya as a 'message signed with blood to the nations of the cross'.

EGYPT has launched seven airstrikes against Islamic State targets in Libya after the extremist group released a grisly video showing the beheading of 21 Coptic Christians it had held hostage for weeks.

A spokesman for the Armed Forces General Command announced the strikes on state radio Monday, marking the first time Cairo has publicly acknowledged taking military action in neighboring Libya, which has been beset by militia violence for months.

The statement said the warplanes targeted weapons caches and training camps before returning safely. It said the strikes were “to avenge the bloodshed and to seek retribution from the killers.”

“Let those far and near know that Egyptians have a shield that protects them,” it said.

State television showed footage of Egyptian fighter jets it said were taking off to conduct the strikes.

“The air strikes hit their targets precisely, and the falcons of our air forces returned safely to their bases,” the military’s statement said.

“We affirm that avenging Egyptian blood and retaliating against criminals and killers is a duty we must carry out.” The air strikes came hours after the jihadists posted gruesome footage of the beheadings on the Internet, sparking outrage in Egypt and demands for revenge.

A visibly angry President Abdel Fattah al-Sisi said Egypt “reserves the right to respond in a suitable way and time” in a televised speech, and declared seven days of mourning.

United Arab Emirates Foreign Minister Sheikh Abdullah bin Zayed al-Nahyan said the killings underlined the need for greater support for Libya’s beleaguered internationally recognised government, which was ousted from the capital Tripoli last August.

The UAE “supports, with all its capabilities, Egypt’s efforts in eradicating terrorism and violence directed at its nationals and affirms its position in standing alongside and its complete solidarity with it,” Sheikh Abdullah said.

“The criminals must strongly and decisively receive the punishment they deserve without any hesitancy,” he added.

The graphic video showing 21 Egyptian Christians being beheaded in Libya as a “message signed with blood to the nations of the cross”.

“We will conquer Rome,” one of the executioners warns after committing the bloody act.

“The sea you have hidden Osama bin Laden’s body in, we will mix it with your blood”.

Unlike previous videos from Syria and Iraq, this video — released early this morning Australian time — reveals the brutal moment the hostages were killed.

“All crusaders: safety for you will be only wishes, especially if you are fighting us all together. Therefore we will fight you all together,” he said.

The footage released online shows handcuffed hostages wearing orange Guantanamo-Bay style jumpsuits, each accompanied by a masked militant.

The men are made to kneel and one militant addresses the camera in English before the men are simultaneously beheaded on the seashore near the Libyan capital of Tripoli.

The water is then shown stained red with their blood.

The makers of the video identify themselves as the Tripoli Province of the Islamic State group — the Islamic militant group that controls about a third of Syria and Iraq.

The video, titled “A message signed with blood to the nation of the cross”, has a scrolling caption in the first few seconds saying it is directed at “People of the cross, followers of the hostile Egyptian Church”.

This morning’s video comes just days after IS released a video showing the gruesome burning alive of a Jordanian pilot it captured after his F-16 came down in Syria in December.

The highly choreographed video showing the killing of Maaz al-Kassasbeh triggered global outrage.

In January, the IS branch in Libya claimed it had abducted 21 Christians. In the latest issue of the IS online magazine Dabiq, the group said 21 Egyptian hostages were being held, and pictures showed a similar background.

A spokesman for the Egyptian foreign ministry confirmed in Cairo that 20 Egyptians had been kidnapped in two separate incidents in neighbouring Libya.

Badr Abdelatty did not say when they were seized or specify their religious affiliation, but said seven Egyptians and 13 others abducted separately in Libya “are still being detained” by their captors.

IS beheaded eight Egyptians inside Egypt last month.

The Egyptian government declared a seven-day mourning period and President Abdel Fattah el-Sissi convened an emergency national security meeting to discuss a response. El-Sissi sent condolences to “the victims of terrorism,” according to a statement released by the presidency.

Qalini Sanyout, whose two nephews were among the hostages, answered the phone Sunday night amid the sound of wailing in the background.

“What can we say? Can someone tell us if this is true.” he said repeatedly. “The whole village is in mourning. Men are covering their heads with dust and mud.”