Cairo: Egyptian warplanes bombed sites in Libya on Monday, a day after Islamic State (Isis) militants released a video depicting the apparent mass beheading of 21 Egyptian Coptic Christians.

The ISIS militants had held the Christians hostages for weeks.

A spokesperson for the Armed Forces General Command announced the strikes on state radio on Monday. This was the first time Cairo publicly acknowledged taking military action in neighboring Libya, where extremist groups seen as a threat to both countries have taken root in recent years, report agencies.

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. 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.”

Meanwhile Libya’s air force announced it had launched strikes in the eastern city of Darna, which was taken over by an Islamic State affiliate last year.

The extremist group controls about a third of Syria and Iraq in a self-declared caliphate and the killings of Christians raise the possibility it has established a direct affiliate less than 800 km from the southern tip of Italy. One of the militants in the video said the group now plans to “conquer Rome.”

The militants had been holding 21 Egyptian Coptic Christian laborers rounded up from the city of Sirte in December and January.

It was one of the first such beheading videos 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.

French President François Hollande and Abdel Fatah al-Sisi, his Egyptian counterpart, called on Monday for the UN security council to meet over Libya and to take new measures. The two had discussed the situation over telephone, Hollande’s office said.

Egypt has provided clandestine support to forces allied to Libya’s internationally recognized government, which fled the capital for the eastern cities of Bayda and Tobruk last year. According to western officials, Egypt also allowed its bases to be used for air strikes in Libya in 2014. Egyptian government officials denied such military involvement.