Egyptian President Abdel Fattah al-Sisi has said his country reserves the right to retaliate against the Islamic State (Isis) for beheading 21 Coptic Christians in Libya.

In a televised address to the nation, the former army general said Egypt will respond to IS's killings at a suitable place and time.

Sisi added that the National Defence Council was drafting a plan on further action against the Islamist group.

Over the weekend, the Sunni insurgent group released a video which appeared to show the beheading of 21 Egyptian Christians, who were recently abducted in Libya. Dressed in orange overalls, the victims were shown being decapitated by a group of masked men.

The footage makes it clear that the Christians were beheaded because of their faith as a scrolling caption reads: "People of the cross, followers of the hostile Egyptian Church."

"Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals," said the Egyptian president.

"Egypt reserves the right to respond in a suitable way and time to punish these murderers."

Egypt's border security has been tightened following the execution as Sisi ordered officials to extend maximum help to those Egyptians who want to flee Libya.

A seven-day mourning period has also been declared.

Al-Azhar, the Egypt-based Islamic learning group, has denounced the killings.

"Al-Azhar stresses that such barbaric action has nothing to do with any religion or human values," an Al-Azhar statement said.

Libya has descended into chaos ever since former dictator Muammar Gaddafi was ousted n 2011 as Islamist militias continue to capitalise on the volatile situation.

Though the IS claims to have carried out a string of attacks in the crisis-torn Libya, it is still unclear how powerful the organisation is in Libya amid the presence of other Islamist groups.