A video showing the mass beheading of Egyptian Coptic Christian hostages by Islamic State (IS) linked terrorists in Libya has sparked worldwide revulsion.
The United States described the act as "despicable" and "cowardly" and White House press secretary Josh Earnest said the militant group's barbarity "knows no bounds".
Mr Earnest said the killings underscored the need for a political resolution to the conflict in Libya. He added the situation there benefited only terrorist groups and called on the Libyan people to unite in opposition to terrorism.
In the UK, Foreign Secretary Philip Hammond said: "Such barbaric acts strengthen our determination to work with our partners to counter the expanding terrorist threat to Libya and the region.
"Acts of terrorism should not be allowed to undermine Libya's political transition.
"We remain fully supportive of the UN's efforts to build a national unity government for Libya and to bring a political solution to the ongoing security crisis. Those who support terrorists can have no part in this process."
The killings raise the possibility that IS - which controls about a third of Syria and Iraq in a self-declared caliphate - has established a direct affiliate less than 500 miles from the southern tip of Italy.
One of the militants in the video makes direct reference to that possibility, saying the group now plans to "conquer Rome".
The terrorists had been holding 21 Egyptian Coptic Christians - labourers rounded up from the city of Sirte in December and January - hostage for weeks. It was not clear from the video whether all 21 were killed.
It was one of the first such beheading videos from an IS affiliate to come from outside the group's core territory in Syria and Iraq.
The Egyptian government and the Coptic Church, which is based in Egypt, both declared it authentic.
The Egyptian government declared a seven-day mourning period and President Abdel Fattah el-Sissi addressed the nation, pledging resilience in the fight against terrorism.
"These cowardly actions will not undermine our determination" said Mr el-Sissi, who also banned all travel to Libya by Egyptian citizens and said his government reserved the right to seek retaliation.
"Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals."
The Coptic Church called on its followers to have "confidence that their great nation won't rest without retribution for the evil criminals".
The video's makers identified themselves as the Tripoli Province of the Islamic State. A still photo, apparently taken from the video, was published last week in the IS's Dabiq online magazine - indicating a direct connection between the Libyan militants and the main group.
The video depicts several men in orange jumpsuits being led along a beach, each accompanied by a masked militant. The men are made to kneel and one militant, dressed differently than the others, addresses the camera in North American-accented English.
"All crusaders: safety for you will be only wishes, especially if you are fighting us all together. Therefore we will fight you all together," he said. "The sea you have hidden Sheikh Osama Bin Laden's body in, we swear to Allah we will mix it with your blood."
The men are then laid face-down and simultaneously beheaded.
The militant speaker then pointed northwards across the red-stained waves and said: "We will conquer Rome, by Allah's permission."
In el-Aour, a dusty and impoverished village south of Cairo and home to 13 of the hostages, the community was wrapped in sorrow. Men covered their heads with dirt in a sign of both grief and shame and women slapped their own faces or let out heart-wrenching shrieks of pain.
Villagers accused the Egyptian government of doing little to help the captives. The authorities, they say, were able to free Muslim Egyptians abducted in Libya in recent months but have done nothing to save the 21 because they are Christian - an accusation rooted in the deep sense of religious discrimination felt by most Egyptian Copts.
Samuel Walham's family immediately recognised him from the picture, showing him kneeling on the beach alongside four other hostages - each flanked by a knife-wielding militant.
"Look at my love. Look how beautiful he is," his mother, Ibtassal Lami, said through tears as she cradled a photo of her son and women wailed in the family's ramshackle, two-storey home. "He only went there to earn his living."
Libya, rich in oil and short on labour, has long been a magnet for Egyptians from all professions. Labourers have flocked there to escape poverty and unemployment at home, while professionals have gone in search of a better salary. Libya's 2011 civil war left much of the country in ruins, creating a boom for skilled foreign workers.
Egyptians are the largest single group of foreign workers in Libya, but over time, the risks have grown. Egyptians, and Copts in particular, have become frequent targets for Islamic extremists who have flourished amid Libya's political turmoil.
Islamic and tribal militias have overrun Libya's two largest cities, Tripoli and Benghazi - forcing the elected Western-based government and parliament to meet elsewhere.
Egyptian authorities have responded by suspending most flights to Libya and issuing travel warnings. Yet, Egyptian workers remain undeterred and still line up outside the Libyan embassy in Cairo in search of visas.