Egyptian jets bombed Islamic State targets in Libya yesterday, a day after the group there released a video showing the beheading of 21 Egyptian Christians, drawing Cairo directly into the conflict across its border.

Egypt said the dawn strike hit militant camps, training sites and weapons storage areas in neighbouring Libya.

Libya's air force also participated in Monday's attack on Derna -- an eastern coastal city seen as a base for Islamic State fighters in the oil-rich nation.

The rival Tripoli-based parliament, which is supported by some Islamist groups, condemned yesterday's strike as an assault on the country's sovereignty.

The 21 Egyptian Coptic Christians, who had gone to Libya in search of work, were marched to a beach, forced to kneel and then beheaded on video, which was broadcast via a website that supports Islamic State.

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." Afterwards, he says: "And we will conquer Rome, by the will of Allah."

The head of the Roman Catholic Church, Pope Francis, condemned the killings. "They were killed simply for the fact that they were Christians," he said at the Vatican. "It makes no difference whether they be Catholics, Orthodox, Copts or Protestants. They are Christians!"

Egypt's Coptic Christian pope was one of the public figures who backed Sisi when he, as army chief, ousted Islamist president Mohamed Mursi in 2013 after mass protests against him.

The beheadings could pile pressure on Sisi to show he is in control of Egypt's security, even though he has already made progress against Islamist militant insurgents in the Sinai.

Egypt has been trying to project an image of stability ahead of an investment conference in the Red Sea resort city of Sharm el-Sheikh in March designed to lure billions of dollars into an economy battered by turmoil since the 2011 uprising.