Egypt's military has bombed targets in Libya, a day after a video showing the beheading of 21 Egyptian Coptic Christians was released by Islamic State-inspired combatants.
The attack, in which Libya's air force also took part, focused on camps, training sites and weapons storage areas across the border between the two countries.
A Libyan air force commander told Egyptian state television that 40-50 militants were killed, while two security officials said civilians, including three children and two women, died.
The strikes come after video emerged showing handcuffed Coptic hostages dressed in orange jumpsuits being murdered on a beach by militants calling themselves the Tripoli Province of Islamic State.
The Egyptian victims had travelled to Libya for work and were kidnapped in two groups in December and January from the coastal city of Sirte.
In the video a camouflaged fighter points north across the Mediterranean and threatens in an American accent that IS, which already controls around a third of Iraq and Syria, will soon reach Europe.
Saqer al Joroushi of the Libyan air force said the strikes by war planes loyal to the official government had been coordinated with Egypt and more would follow.
He said Egyptian and Libyan planes had combined to hit targets in the eastern town of Derna. Libyan planes then attacked the central cities of Sirte and Ben Jawad.
Egyptian planes later returned for a second wave of airstrikes on Derna.
A spokesman for Egypt's Armed Forces General Command said the raids were "to avenge the bloodshed and to seek retribution from the killers".
"Let those far and near know that Egyptians have a shield that protects them," the statement added.
Egypt's Copts are the largest Christian community in the Middle East and are estimated to account for around 15% of the country's population.
Egypt's ambassador to the UK, Nasser Kamel, told Sky News nations need to join together and face the IS threat, which is "coming closer to home, and by home I mean not only Egypt, but also Europe".
Britain's Prime Minister David Cameron described the killings as "a simply barbaric and inhumane act", and warned Libya must not become a "safe haven for terrorists".
In the aftermath, Egypt's president, Abdel Fattah al Sisi, threatened to carry out a "suitable" punishment.
Foreign Minister Sameh Shoukry said "tough intervention" against the militants in Libya is needed or they would threaten world peace.
French President Francois Hollande and Mr al Sisi have urged the UN Security Council to meet and consider new measures to tackle the militants.
Meanwhile, Libya's internationally recognised prime minister, Abdullah al Thinni, has called on the West to launch airstrikes on militants in his country.
The country is split between rival governments backed by different militias: Mr al Thinni's, which is based in Tobruk, and another located in Tripoli.
The Tripoli-based parliament strongly condemned the Egyptian strike on Derna as an "assault against Libyan sovereignty".
