Islamic State released a video yesterday purporting to show the militant group beheading 21 Egyptian Christians kidnapped in Libya.

In the video, militants in black marched the captives, dressed in orange jump suits, to a beach. They were forced down onto their knees, then beheaded. The video 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.”

Thousands of Egyptians have travelled to neighbouring Libya in search of jobs since an uprising at home in 2011, despite advice from their government not to go to one of the most dangerous countries in the region.

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.”

Meanwhile officials of Libya’s rival governments have been in touch over the growing threat of Islamic State militants, a top Libyan security official said, suggesting the fight against a common enemy might help to unite the country’s warring factions. Two separate governments are vying for control of Libya, four years after a Nato-backed civil war ousted Muammar Gaddafi. Western governments worry that Islamist militants are exploiting the discord. Several groups in Libya have declared their allegiance to Islamic State, whose jihadists have taken over large swathes of Iraq and Syria they claim as their own Islamic caliphate.

Fighters claiming ties to Islamic State in Libya said they carried out an attack on Tripoli’s Corinthia Hotel last month, when gunmen shot nine people, including an American security adviser and a Frenchman.

“Islamic State is a big, big threat,” said Omar al-Zanki, the interior minister in the official government. Prime Minister Abdullah al-Thinni suspended Zanki for criticism of former general Khalifa Haftar, but al-Zanki said he continues to command troops pending the outcome of a legal complaint over his case.

Zanki blamed supporters of Islamic State in Iraq and Syria for the storming of government buildings in the central city of Sirte this week. State media had made the incident public yesterday without releasing details.