Egypt carried out air strikes against Islamic State group targets in Libya on Monday after the jihadists posted a video showing the decapitation of 21 Coptic Christians.
The air strikes in the early morning hit IS camps and stores of weapons, the military said, hours after the jihadists released the gruesome footage of the beheadings.
Witnesses told AFP 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.
It was the first time Egypt announced military action against Islamist targets in its western neighbour, having previously denied it targeted militants there.
"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.
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 President Abdel Fattah al-Sisi threatened a "suitable response" to the killings of the Christians who had travelled to Libya seeking work.
Sisi, a former army chief who overthrew the Islamist president and then won elections touting his firm hand, faced a chorus of demands to retaliate after the beheadings.
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.
'Unite against' IS
Images taken from the video of waves breaking on the beach turning red from the blood of the victims spread among Egyptians on social media, enraging many.
The White House led condemnation of the apparent beheadings, describing the killers as "despicable" and adding that the brutality shown "further galvanises the international community to unite against ISIL," an alternative acronym for the group.
French President Francois Hollande, whose government is poised to sign a deal selling Egypt advanced Rafale fighter jets on Monday, expressed his "concern at the expansion of Daesh in Libya," using another name for IS.
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 Coptic Church issued a statement saying it was "confident" the Christians' killers would be brought to justice, as it confirmed those beheaded were Egyptian Copts.
Al-Azhar, the prestigious Cairo-based seat of Islamic learning, denounced the "barbaric" killings.
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 IS branch in Libya had claimed in January to have abducted 21 Christians.
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.
It says the beheadings were in revenge for the treatment of Shehata, who Islamists claim was held against her will by the Church following her conversion.
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.