Islamic State released a video on Sunday purporting to show the beheading of a group of Egyptian Christians kidnapped in Libya, violence likely to deepen Cairo's concerns over security threats from militants thriving in the neighboring country's chaos.
Egypt's state news agency MENA quoted the spokesman for the Coptic Church as confirming that 21 Egyptian Christians believed to be held by Islamic State were dead.
In the video, militants in black marched the captives, dressed in orange jump suits, to a beach the group said was near Tripoli. They were forced down onto their knees, then beheaded.
The video appeared on the Twitter feed of a website that supports Islamic State, which has seized parts of Iraq and Syria and has also beheaded Western hostages.
A caption on the five-minute video read: "The people of the cross, followers of the hostile Egyptian church."
Thousands of Egyptians have traveled to Libya in search of jobs since an uprising at home in 2011, despite advice from their government not to go to a country sliding into lawlessness.
Before the killings, one of the militants stood with a knife in his hand and said: "Safety for you crusaders is something you can only wish for."
President Abdel Fattah al-Sisi called a seven-day mourning period and an urgent meeting of Egypt's top military commanders, state television reported.
The Coptic Church said it was confident the Cairo government would seek justice. Al Azhar, the center of Islamic learning in Egypt, said no religion would accept such "barbaric" acts.
The families of the kidnapped workers had urged Cairo to help secure their release. In the southerly Minya Governorate, relatives screamed and fainted upon hearing news of the deaths.
CONCERNS ABOUT LIBYA
Sisi has repeatedly expressed concerns about militants based in Libya who are seeking to topple his government.
Those militants have made contact with Sinai Province, a group operating from Egypt's Sinai Peninsula that has changed its name from Ansar Beyt al-Maqdis and pledged allegiance to Islamic State.
The group has killed hundreds of Egyptian soldiers and police since the army toppled Islamist president Mohamed Mursi in 2013 after mass protests against his rule.
With Libya caught in a chaotic power struggle between two rival factions operating their own governments, Western officials fear Islamist militants are taking advantage of the turmoil to strengthen their presence.
A number of Islamist militant groups have been active since the fall of Muammar Gaddafi in 2011 left Libya without a strong central government. A few have declared ties to the radical Islamic State and claimed high-profile attacks over recent weeks in what appears to be an intensifying campaign.
Last month, Islamic State claimed responsibility when at least two gunmen stormed into the five-star Corinthia Hotel in Tripoli, killing nine people, including an American security contractor and a Frenchman.
Fears that the crisis in neighboring Libya could spill across the border have prompted Egyptto upgrade its military hardware. French President Francois Hollande has said Egypt will order 24 Rafale fighter jets, a naval frigate and related military equipment in a deal to be signed in Cairo on Monday worth more than 5 billion euros ($5.7 billion).
