Islamic State has released a video purporting to show the beheading of a group of Egyptian Christians kidnapped in Libya. 

Egypt's state news agency MENA quoted a spokesman for the Coptic Church as confirming that 21 Egyptian Christians were dead.             

In the video, militants in black are seen marching the group, all dressed in orange jump suits, to a beach that IS said was near Tripoli.

The captives were forced to their knees and then beheaded.   

President Abdel Fattah al-Sisi called a seven-day mourning period and an urgent meeting of Egypt's top military commanders, state television reported.              

The Coptic Church said it was confident the Cairo government would seek justice.

Al Azhar, the centre of Islamic learning in Egypt, said no religion would accept such "barbaric" acts.             

Relatives of the kidnapped workers had urged Cairo to help secure their release.

In the southerly Minya Governorate, relatives screamed and fainted upon hearing news of the deaths.         

The video appeared on the Twitter feed of a website that supports Islamic State, which has seized parts of Iraq and Syria and has also beheaded Western hostages.             

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 a country sliding into lawlessness.             

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."            

President Sisi has repeatedly expressed concerns about militants based in Libya who are seeking to topple his government.

Those militants have made contact with Sinai Province, a group operating from Egypt's Sinai Peninsula that has pledged allegiance to Islamic State.             

Sinai Province has killed hundreds of Egyptian soldiers and police since the army toppled Islamist president Mohammed Mursi in 2013 after mass protests against his rule.             

Last month, Islamic State claimed responsibility when at least two gunmen stormed into the five-star Corinthia Hotel in Tripoli, killing nine people, including a US security contractor and a Frenchman.