Egypt’s President Abdel el-Sisi said Cairo “reserves the right to respond in any way” to the killing of 21 Copts by Islamic State militants. The extremist group has released a graphic video showing the beheadings of the Copts that were kidnapped in Libya.

Sisi warned that Cairo would choose the “necessary means and timing to avenge the criminal killings” as he spoke on state television in the wake of the surfaced footage of the brutal executions.

The Copts – native Christians of Egypt – are shown in the video dressed in orange jumpsuits with their hands tied behind their backs, while being walked along the seaside by jihadists dressed in black. The Egyptian men are then forced to kneel before being beheaded.

Egypt's Sisi: "Reserves Right of Retaliation"-After Coptic Christians beheaded by ISIS Libya http://t.co/oEaTa2ic0Tpic.twitter.com/tkaaEanwJ9 — Tom T. (@VRWCTexan) February 16, 2015

Several very graphic images of the killings appeared on Twitter. RT will not run the images, believing it is unethical to publish the footage or pictures of atrocities committed by terrorists even if they cannot be immediately verified.

A caption on the five-minute video read: "The people of the cross, followers of the hostile Egyptian church."

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."

The Coptic Church confirmed that 21 Egyptian Christians believed to have been held by IS are dead, Egypt's state news agency MENA reported.

It is often the poorest Egyptians who go to #Libya to find work,despite risks&civil war.The 21 killed all came from Samalout,Minya,all poor. — Bel Trew - بل ترو (@Beltrew) February 15, 2015

Following the release of the video, Sisi called urgent security talks. Egypt has announced seven days of mourning.

The Egyptian Foreign Affairs Ministry banned travel to Libya following the killings on Sunday.

The IS militants first published the images of the 21 Copts on Thursday, in the group’s Dabiq magazine. The men were reportedly abducted in the Libyan city of Sirte on two separate occasions – seven on December 31 and 14 on January 3.

READ MORE: ISIS takes credit for abduction of 21 Christians in Libya

Later on Thursday, Cairo began to evacuate all Egyptian citizens wishing to return from Libya, and has renewed a travel alert to the neighboring country. At the same time, the Egyptian presidential office issued a statement vowing to return the kidnapped citizens.

Militants from IS – an Al-Qaeda offshoot group – control territory in Iraq and Syria and operate in other unstable regions of the Middle East, North Africa, and Asia, such as in eastern Libya and the Sinai Peninsula of Egypt.

The group has previously performed similar executions of Jordanian, Japanese, American, British, and French hostages, which were also filmed and published on social media. Previously known as ISIL/ISIS, the Islamic State is also said to have executed hundreds of civilians in militant-controlled areas of Syria and Iraq.

READ MORE: ‘Crucify them!’ Muslim leaders condemn ISIS execution of Jordanian pilot

Libya, a once prosperous North African nation, descended into chaos when NATO-supported rebel forces took up arms to oust former leader Muammar Gaddafi in 2011. Thousands of Egyptians come to Libya for work, primarily in the construction sector. Scores of Egyptian Christians have been targeted by militants in the unstable country.

Last February, the bodies of seven Coptic Christians were found near Benghazi, which is partially held by Islamist militias. In March 2013, dozens of Egyptian Christians – accused by extremists of proselytizing – were tortured in a Benghazi detention center.