A video purporting to show the mass beheading of Coptic Christian hostages has been released by militants in Libya claiming loyalty to the Islamic State (ISIS) group.

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 800 kilometres 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 militants had been holding 21 Egyptian Coptic Christians hostage for weeks, all labourers rounded up from the 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 such beheading videos from an Islamic State group affiliate to come from outside the group's core territory in Syria and Iraq.

'We will conquer Rome, by Allah's permission.' - Libyan jihadist

The Egyptian government declared a seven-day mourning period and President Abdel Fattah el-Sissi addressed the nation late Sunday night, pledging resilience in the fight against terrorism.

"These cowardly actions will not undermine our determination" said el-Sissi, who also banned all travel to Libya by Egyptian citizens and said his government reserves 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 in a statement 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 group. A still photo, apparently taken from the video, was published last week in the Islamic State group's Dabiq online magazine — indicating a direct connection between the Libyan militants and the main group.

The video, released late Sunday night, depicted 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 that 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 northward and said, "We will conquer Rome, by Allah's permission."

Militants in Libya had been holding 21 Egyptian Coptic Christians hostage for weeks, threatening them with death.

'The whole village is in mourning. Men are covering their heads with dust and mud ' - Qalini Sanyout, uncle of two of the hostages

The makers of the video identify themselves as the Tripoli Province of the Islamic State group — the Islamic militant group that controls about a third of Syria and Iraq. The Associated Press could not immediately independently verify the video.

In Washington, White House press secretary Josh Earnest called the Islamic State group "despicable" and "cowardly,"
adding the group's barbarity "knows no bounds."

Qalini Sanyout, whose two nephews were among the hostages, answered the telephone Sunday night amid the sound of wailing in the background.

"What can we say? Can someone tell us if this is true." he said repeatedly. "The whole village is in mourning. Men are covering their heads with dust and mud."

Mideast Egypt Libya

The beheading of 21 Coptic Christian men by ISIS in Libya earlier in the year caused outrage in neighbouring Egypt. (Hassan Ammar/Associated Press)