Islamic State has released a video purporting to show the militant group beheading 21 Egyptian Christians kidnapped in Libya.
The video, released this afternoon, shows ISIS fighters dressed head to toe in black, marching the captives, all wearing orange jumpsuits, to a beach.
The 21 men can be seen being forced onto their knees before they are beheaded by the militants standing behind them
Horror: A new video has been released by ISIS allegedly showing the beheading of 21 Egyptian men
The extremely graphic video first appeared on the Twitter feed of a website that supports Islamic State.
A caption on the five-minute video read: 'The people of the cross, followers of the hostile Egyptian church.'
Before the killings, one of the militants stood with a knife in his hand and said: 'Safety for you crusaders is something you can only wish for.'
Egypt has vowed to avenge the brutal murders, which have been condemned around the globe, including by British Foreign Secretary Philip Hammond.
Mr Hammond said in a statement: 'I strongly condemn the murders of 21 Egyptian Coptic Christians by ISIL-affiliated extremists in Libya. My thoughts are with the families of those killed.
Terrorists: The extremely graphic video first appeared on the Twitter feed of a website that supports ISIS
'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.'
Earlier tonight, President Abdel Fattah al-Sisi said that Egypt reserved the right to respond in a way it sees fit to the Islamic State's beheading of 21 Egyptians in neighbouring Libya.
Sisi warned Cairo would choose the 'necessary means and timing to avenge the criminal killings'.
'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 Coptic Church in a statement called on it followers to have 'confidence that their great nation won't rest without retribution for the evil criminals.'
Helpless: Images emerged on Saturday, allegedly showing ISIS militants executing two men accused of spying for the Syrian government
Militants in Libya had been holding the 21 Egyptian Coptic Christians hostage for weeks, all laborers rounded up from the city of Sirte in December and January.
The makers of the video identify themselves as the Tripoli Province of the Islamic State group.
Libya has seen a rise of powerful armed militias in the wake of the downfall and death of dictator Muammar Gaddafi in the 2011 NATO-backed uprising.
Tens of thousands of Egyptians work in Libya, mainly in the construction sector, and they have been targeted as the country has descended into chaos.
The killings raise the possibility that the Islamic militant group - 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, speaking English, makes direct reference to that possibility, saying the group now plans to 'conquer Rome.'
Yesterday, images emerged of ISIS militants publicly executing two alleged spies and mercilessly crucifying another to a road sign in the Syrian city of Al-Bab.
On Thursday, the extremist group announced it was holding an Israeli-Arab who posed as foreign a fighter to spy for the country's intelligence agency, Mossad.