A video has emerged showing the beheadings purportedly of 21 Egyptian Christians who had been kidnapped by Islamic State (IS) militants in Libya.
The footage shows a group wearing orange overalls, being forced to the ground and then decapitated.
Egypt’s National Defence Council is to meet in emergency session to discuss its response to the killings.
IS militants claim to have carried out several attacks in Libya, which is effectively without a government.
However, with many armed groups operating in Libya, it is not clear how much power IS actually wields.
The kidnapped Egyptian workers, all Coptic Christians, were seized from the coastal town of Sirte in eastern Libya, now under the control of Islamist groups.
On Friday, IS released pictures of the Egyptians, saying they had been kidnapped to avenge the fate of Muslim women “tortured and murdered by the Coptic church of Egypt”.
The video of the beheadings was posted online by Libyan jihadists who pledge loyalty to IS.
The identities of the murdered men have not been confirmed, but Egypt has declared seven days of national mourning.
Libya has been in turmoil since 2011 and the overthrow of its then-leader, Col Muammar Gaddafi.
Since then, numerous other militia groups have battled for control.
The head of the US Defense Intelligence Agency warned last month that IS was assembling “a growing international footprint that includes ungoverned and under-governed areas”, including Libya.
Libya has two rival governments, one based in Tripoli, the other in Tobruk. Meanwhile, the eastern city of Benghazi, headquarters of the 2011 revolution, is largely in the hands of Islamist fighters, some with links to al-Qaeda
Many Egyptians go to Libya looking for work, despite warnings from their government.
Egypt’s president had earlier this week offered to airlift Egyptian expatriates out of Libyan territory.
There had been demonstrations in Egypt calling on the government to do more to secure the release of those held.
Estimates as to the number of Copts living in Egypt today range from about nine million to 15 million out of a total population of about 87 million.