The 21 — mainly young men from impoverished families — had travelled to Libya for work and were kidnapped in two groups in December and January from the coastal city of Sirte. In the video, the group is marched onto what is purported to be a Libyan beach before masked militants with knives carve off the head of each. The killing of at least a dozen of them is clearly visible, though it was not clear from the video whether all 21 hostages were killed.Two rounds of Egyptian airstrikes, several hours apart on Monday, struck targets in the eastern Libyan city of Darna, according to Egyptian and Libyan security officials, who spoke on condition of anonymity because they were not authorized to talk the press.
Egypt's military announced the strikes on state radio, marking the first time Cairo has publicly acknowledged military action in Libya. It said the strikes hit weapons caches and training camps "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 cuts off terrorism," it said.
Libya's air force commander, Saqr al-Joroushi, told Egyptian state TV that the airstrikes were coordinated with the Libyan side. Libya's air force said it also carried out strikes in Darna.
Libya has become home of the strongest presence of the Islamic State group outside its core territory in Syria and Iraq.
With almost no state control in much of Libya, extremists loyal to the Islamic State have seized control Darna and the central city of Sirte and have built up a powerful presence in the capital Tripoli and the second-largest city Benghazi. Libyan Interior Minister Omar al-Sinki has said some 400 militants from Yemen and Tunisia are believed to make up the group in Libya, along with Libyan militias that have vowed allegiance.
Egypt appears now to be launching a push for international military intervention in Libya to curb the group.
Egyptian President Abdel-Fattah el-Sissi spoke with France's president and Italy's prime minister Monday about the Libya situation. He sent his foreign minister, Sameh Shukri, to New York to consult with U.N. 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.
Egypt's Foreign Ministry said the international community must adopt "immediate and effective" moves against terror groups in Libya. "Leaving things in Libya as they are without decisive intervention to suppress these terror groups constitutes a clear danger to international peace and security," it said.
It also called on the U.S.-led coalition staging airstrikes against the Islamic State group in Iraq and Syria to offer Egypt political and material support to counter the group in Libya.
The idea of intervention has gained traction with Italy, whose southern tip is less than 500 miles (800 kilometers) from the Libyan coast. One of the militants in the video boasted the group plans to "conquer Rome."
Italian Defense Minister Roberta Pinotti said in an interview published Sunday in the Il Messaggero daily that her country was ready to lead a coalition of countries — European and North African — to stop the militants' advance.
"If in Afghanistan we sent 5,000 men, in a country like Libya which is much closer to home, and where the risk of deterioration is much more worrisome for Italy, our mission and commitment could be significant, even numerically," she was quoted as saying.
Italy, she said, would gladly take a lead role "for geographic, economic and historic reasons," but she stressed that so far such an intervention was only theoretical. Asked if ground troops were a possibility, she said it would depend on the scenario.
Egypt is already battling an Islamist insurgency in the strategic Sinai Peninsula, where militants have recently declared their allegiance to the Islamic State and rely heavily on arms smuggled from Libya.
Libya, on Egypt's western border, has slid into chaos since the 2011 uprising that toppled and killed longtime leader Moammar Gadhafi.
Islamist-allied militias seized the capital Tripoli last year, and the internationally recognized government has been confined to the country's far east since, while Islamist politicians set up a rival government in Tripoli.
Last year, Egypt and the United Arab Emirates carried out airstrikes against Islamist-allied forces last year, according to U.S. officials.
The foreign minister of the United Arab Emirates, Sheikh Abdullah bin Zayed Al Nahyan, called the new mass killing an "ugly crime" and said it was "devoting all its resources to support the efforts of Egypt to eradicate terrorism and the violence directed against its citizens."
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.
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Michael reported from el-Aour, Egypt. Associated Press writers Nicole Winfield in Rome, Maamoun Youssef in Cairo and Adam Schreck in Dubai, United Arab Emirates contributed to this report.