VATICAN CITY: Pope Francis has denounced the slaying by the Islamic State of Egyptian Copts in Libya, saying they were "assassinated just for being Christian".

Francis spoke off-the-cuff on Monday during an audience with a Scottish ecumenical delegation. He said: "They only said 'Jesus help me ...' The blood of our Christian brothers is testimony that cries out. Be they Catholic, Orthodox, Copts, Lutherans, it doesn't matter: They're Christian!"

Militants in Libya affiliated with the Islamic State group released a video late Sunday purporting to show the mass beheading of Coptic Christian hostages held for several weeks. The killings raised anxieties in Italy, given Libya's proximity just across the Mediterranean Sea and because one of the militants in the video said the group plans to "conquer Rome", the seat of Catholicism.

Egypt strikes Libya jihadists after beheadings video

Cairo launched air strikes against Islamic State group targets in Libya on Monday after the jihadists posted a video showing the decapitation of 21 Egyptian Christians.

Egypt said a “tough intervention” was needed and with France called on the UN Security Council to “take new measures” against the jihadists in neighbouring Libya.

With Monday's air strikes, Egypt opened a new front while already battling IS jihadists in its Sinai Peninsula who have killed scores of troops since the army toppled Islamist president Mohamed Morsi in July 2013.

President Abdel Fattah al-Sisi, the former army chief who toppled Morsi and has been criticised for a deadly crackdown on dissent, has presented Egypt as a key partner in international efforts against the jihadists.

Monday's early morning air strikes hit IS camps and stores of weapons, the military said, hours after jihadists released gruesome footage of the beheadings that provoked outrage in Egypt.

Witnesses told AFP that there were at least seven air strikes in Derna in the east, a hotbed of militancy since Libyan dictator Moamer Kadhafi was overthrown in 2011. It was not immediately clear whether other areas were also hit.

"Your armed forces on Monday carried out focused air strikes in Libya against Daesh camps, places of gathering and training, and weapons depots," the military said in a statement, using the Arabic acronym for IS.

It was the first time Egypt announced military action against Islamist targets in its western neighbour, having previously denied it targeted militants there. But US officials had said that Egypt had previously allowed the United Arab Emirates to use its bases to bomb militants in Libya.

'Avenging Egyptian blood'

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

"Avenging Egyptian blood and retaliating against criminals and killers is a duty we must carry out," the military said.

The air strikes came hours after Sisi threatened a "suitable response" to the killings of the Coptic Christians. Foreign Minister Sameh Shoukry was to head to Washington to take part in an "anti-terrorism" summit, the ministry said, calling for strong international action.

"Leaving matters as they are in Libya without tough intervention to curb these terrorist organisations represents a clear threat to international security and peace," it said.

The brutal deaths of the Christians, who like thousands of poor Egyptians had travelled abroad to seek work, shocked their compatriots.

"Revenge is on the way", read the banner on the front page of the official Al-Akhbar newspaper and both the Coptic Church and the prestigious Islamic Al-Azhar institution condemned the attack.

Egyptian television repeatedly played the video without the beheadings, showing black-clad militants leading their captives in orange jumpsuits along a beach before forcing them to kneel down.

Analysts said attacking the militants underscores Sisi's message that Egypt is a bulwark against a shared jihadist enemy with Western countries that have been critical of his human rights record.

"These strikes elevate Egypt to a new level in confronting extremist organisations," said Mathieu Guidere, an Islamic studies and politics professor at the University of Toulouse.

International condemnation

The White House led condemnation of the beheadings, saying the killers were “despicable” and that the brutality shown "further galvanises the international community to unite against ISIL," using an alternative acronym for the group.

French President Francois Hollande agreed with Sisi that "the Security Council meet and that the international community take new measures to face up to this danger," a statement from Hollande's office said.

France is poised to sign a deal selling Egypt advanced Rafale fighter jets on Monday.

United Arab Emirates Foreign Minister Sheikh Abdullah bin Zayed al-Nahyan said his country "supports with all its capabilities Egypt's efforts in eradicating terrorism".

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

IS militants have been hammered by US-led air strikes in Iraq and Syria after taking over swathes of the two countries and the group has active affiliates in Egypt and Libya.

The video, entitled “A message signed with blood to the nation of the cross”, has a scrolling caption in the first few seconds referring to the hostages as “people of the cross, followers of the hostile Egyptian Church”.

The video makes reference to Egyptian woman Camilia Shehata, the wife of a Coptic priest whose alleged conversion to Islam sparked a sectarian dispute in Egypt in 2010.

The latest IS video comes after the jihadists released footage earlier this month showing the gruesome burning alive of a Jordanian pilot the group captured after his F-16 came down in Syria in December.