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The Egyptian military said Monday that its warplanes have carried airstrikes against so called Islamic State targets in Libya after the extremist group released a grisly video showing the beheading of several Egyptian 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 Libya.

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. 

Libya's air force meanwhile announced it had launched strikes in the eastern city of Darna, which was taken over by an Islamic State affiliate last year. The announcement, on the Facebook page of the Air Force Chief of Staff, did not provide further details. 

The video purporting to show the mass beheading of Coptic Christian hostages was released late Sunday by militants in Libya affiliated with the Islamic State group. 

The killings raise the possibility that the extremist group   which controls about a third of Syria and Iraq in a self-declared caliphate has established a direct affiliate less than 500 miles (800 kilometers) from the southern tip of Italy. 

The militants had been holding 21 Egyptian Coptic Christian laborers rounded up from the city of Sirte in December and January. It was not clear from the video whether all 21 hostages were killed. 

It was one of the first such beheading videos from an Islamic State group affiliate to come from outside the group's core territory in Syria and Iraq. 

Libya in recent months has seen the worst unrest since the 2011 uprising that toppled and killed longtime dictator Moammar Gadhafi, which will complicate any efforts to combat the country's many Islamic extremist groups. 

The internationally recognized government has been confined to the country's far east since Islamist-allied militias seized the capital Tripoli last year, and Islamist politicians have reconstituted a previous government and parliament. 

Egypt has strongly backed the internationally recognized government, and US officials have said both Egypt and the United Arab Emirates have taken part in a series of mysterious airstrikes targeting Islamist-allied forces. 

The Egyptian government declared a seven-day mourning period after the release of the video and President Abdel Fattah el-Sissi addressed the nation late Sunday night, pledging resilience in a fight against terrorism. 

"These cowardly actions will not undermine our determination" said el-Sissi, who also banned all travel to Libya by Egyptian citizens. "Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals." 

The UN Security Council meanwhile "strongly condemned the heinous and cowardly apparent murder in Libya of 21 Egyptian Coptic Christians by an affiliate of the Islamic State of Iraq and the Levant."