The Egyptian army announced in a statement on Monday morning it had conducted air strikes against Islamic State (ISIS) targets in Libya, including training camps and arms depots.
"We believe that avenging the blood of Egyptian, and extracting retribution from the killers and criminals is a duty we have to carry out," the army statement said. “Let those far and near know that Egyptians have a shield that protects them.”
It was the first time Egypt has publicly acknowledged taking military action in neighboring Libya, where extreme Islamist groups have been ascendant since the ouster and elimination of long-time dictator Muammar Qaddafi in 2011.
On Sunday night, President Abdel-Fattah al-Sisi said Egypt reserves the right to retaliate against the killing of 21 Egyptian Copts by ISIS in Libya at the suitable time and place.
Earlier on Sunday, Islamic State had published a graphic video showing the beheading of Coptic Christians it said it had kidnapped recently, and vowing to fight what it described as "crusaders."
The Egyptian statement said the airstrikes were successful in reaching their targets and that the fighter jets returned safely to their home bases.
Egyptian state television aired footage of jets taking off from bases in Egypt.
Between 40 and 50 ISIS men were killed in the joint attacks by the Egyptian and Libyan militaries, according to Saker al-Jarushi, a commander in Libya's air force quoted by Reuters. He added that weapons and communications centers were also hit.
ISIS footage of the murder of the Copts that was released online shows handcuffed hostages wearing orange jumpsuits being beheaded by their black-suited captors on a seashore in the Libyan capital of Tripoli.
In the latest issue of the ISIS online propaganda magazine Dabiq, the group said 21 Egyptian hostages were being held. The video, titled "A message signed with blood to the nation of the cross", has a scrolling caption in the first few seconds saying it is directed at "People of the cross, followers of the hostile Egyptian Church".
Sunday's video comes just days after ISIS released a video showing the gruesome burning alive of a Jordanian pilot it captured after his F-16 fell in Syria in December.