Egyptian warplanes struck Islamic State targets in Libya on Monday in swift retribution for the extremists' beheading of a group of Egyptian Christian hostages on a beach, shown in a grisly online video released hours earlier.
An armed forces spokesperson announced the strikes on state radio, marking the first time Cairo has publicly acknowledged taking military action in neighbouring Libya, where extremist groups seen as a threat to both countries have exploited the chaos following the 2011 uprising against dictator Moammar Gadhafi.
The statement said the warplanes targeted weapons caches and training camps before returning safely. It said the "intense strikes" were "to avenge the bloodshed and to seek retribution from the killers."
"Let those far and near know that Egyptians have a shield to protect and safeguard the security of the country and a sword that amputates terrorism and extremism," it said.
Egypt is already battling a burgeoning Islamist insurgency centered in the strategic Sinai Peninsula, where militants have recently declared their allegiance to the Islamic State and rely heavily on arms smuggled across the porous desert border between Egypt and Libya.
The strikes also come just a month before Egypt is scheduled to host a major donor's conference at a Sinai resort to attract foreign investment needed to revive the economy after more than four years of turmoil following its own 2011 uprising.
Libya's air force commander, Saqr al-Joroushi, told Egyptian state TV that the airstrikes were coordinated with the Libyan side and that they killed about 50 militants. Separately, a Libyan security official told The Associated Press by telephone that Egyptian warplanes struck four IS positions in the eastern city of Darna, an extremist stronghold that was taken over by an Islamic State affiliate last year.
Two Libyan security officials said civilians, including three children and two women, were killed in the strikes. The officials spoke on condition of anonymity because they were not authorised to speak to the media.
Libya's air force meanwhile said it had carried out its own strikes in Darna, without providing further details.
The video purporting to show the mass beheading of the 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 an affiliate less than 500 miles (800 kilometers) from the southern tip of Italy. One of the militants in the video said the group now plans to "conquer Rome."
The militants had rounded up 21 Egyptian Coptic Christian laborers from the coastal 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 beheading videos from an Islamic State group affiliate to come from outside the group's core territory in Syria and Iraq, and displayed the sophisticated techniques used in previous videos.
Libya in recent months has seen the worst unrest since the 2011 uprising that toppled and killed Gadhafi, which will complicate any efforts to combat the country's many Islamic extremist groups.
An image grab taken from Egypt's state-run television station Al-Masriya shows a Egyptian F-16 fighter jet preparing to take off in the early morning from an undisclosed location to conduct air strikes against Islamic State (IS) group targets in Libya. (AFP Photo)
The internationally recognised 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 that both Egypt and the United Arab Emirates took part in a series of mysterious airstrikes targeting Islamist-allied forces last year.
The Egyptian government declared a seven-day mourning period after the release of the video, and a visibly angry el-Sissi addressed the nation late Sunday night. He also dispatched his foreign minister, Sameh Shukri, to New York to hold consultations with UN officials and Security Council members ahead of a conference on terrorism opening Wednesday in Washington.
"What is happening in Libya is a threat to international peace and security," said el-Sissi, who also banned all travel to Libya by Egyptian citizens.
"These cowardly actions will not undermine our determination," he said. "Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals."
On Monday, el-Sissi visited the main Coptic Cathedral of St. Mark in Cairo to offer his condolences, according to state TV.
The U.N. 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," it said in a statement.
The foreign minister of the United Arab Emirates, Sheikh Abdullah bin Zayed Al Nahyan, also condemned the mass killing, calling it an "ugly crime."
"The United Arab Emirates is devoting all its resources to support the efforts of Egypt to eradicate terrorism and the violence directed against its citizens," he said.
He added that the killing highlights the need to help the Libyan government "extend its sovereign authority over all of Libya's territory."
The oil-rich Emirates, along with Saudi Arabia and Kuwait, has given billions of dollars in aid to Egypt since el-Sissi, who was then military chief, overthrew Islamist President Mohammed Morsi in July 2013 amid massive protests against his yearlong rule.
Egypt has since waged a sweeping crackdown against Morsi's Muslim Brotherhood group, which it has officially branded a terrorist organisation. El-Sissi has insisted the crackdown in Egypt, as well as support for the government in Libya, is part of a larger war on terror.
Video: Egyptian warplanes attack IS targets in Libya