A video has emerged apparently showing the decapitations of 21 Egyptian Christians who had been kidnapped by Islamic State (IS) militants in Libya.
According to a BBC report, the footage shows the victims wearing orange overalls being forced to the ground and then decapitated.
President Abdel Fattah al-Sisi has said Egypt reserves the right to respond in any way it sees fit.
IS militants claim to have carried out several attacks in Libya, where large areas remain ungoverned since the overthrow of its former leader, Col. Ghadafi.
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 in December and January from the coastal town of Sirte in eastern Libya, now under the control of Islamist groups.
The video of the beheadings was posted online by Libyan jihadists who pledge loyalty to IS. A caption made it clear the men were targeted because of their faith.
“Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals,” President Sisi said.
The beheadings were described as “barbaric” by al-Azhar, the highly regarded theological institution which is based in Egypt.
The Coptic church said it was “confident” Egypt would exact retribution. Egypt has declared seven days of national mourning.