At least 40 suspected Islamic State (IS) militants in Libya were killed early Monday in Egyptian airstrikes in revenge for the killing of 21 Egyptian Coptic Christians who had been kidnapped in the Libyan city of Sirte, media reported.

In a statement after the operation in Libya, the Egyptian Armed Forces spokesman said the warplanes struck the group's camps, places of gathering and training, and weapons depots in Libya, Xinhua news agency reported citing state-run Nile TV.

"Let those far and near know that Egyptians have a shield to protect and safeguard the security of the country and a sword that amputates terrorism and extremism," the spokesman added.

It is the first time Egypt has publicly confirmed launching airstrikes against the group in neighbouring Libya, where extremist groups, threatening both countries, have grown rapidly following the ouster of Muammar Gaddafi in 2011.

In a televised speech hours after the release of the IS video late Sunday, Egyptian President Abdel Fattah al-Sisi said Egypt would choose the "necessary means and timing to avenge the criminal killings".

After the early morning Libya operation Monday, Sisi paid a visit to the Coptic Orthodox Cathedral in Cairo to mourn the victims. Egypt Sunday declared a seven-day period of mourning for the victims.

Libya's air force, meanwhile, announced that it had launched strikes in the eastern city of Darna, an extremist base that was taken over by an IS affiliate last year.