Egypt says it has bombed Islamic State targets in Libya, hours after the group published video showing the apparent beheadings of 21 Egyptian Christians.
State TV said the dawn strikes had targeted camps, training sites and weapons storage areas.
Earlier, President Abdel Fattah al-Sisi said Egypt had the “right to respond” against IS.
A video emerged on Sunday showing a group wearing orange overalls being forced to the ground and decapitated.
IS militants claim to have carried out several attacks in Libya, which is in effect without a government.
The kidnapped Egyptian workers, all Coptic Christians, were seized in December and January from the coastal town of Sirte in eastern Libya, under the control of Islamist groups.
The video of the beheadings was posted online by Libyan jihadists who pledge loyalty to IS.
The killings were described as “barbaric” by al-Azhar, the highly regarded theological institution which is based in Egypt.
Egypt’s military said Monday’s air strikes 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,” it said.
The statement said all planes involved in the operation had returned safely.
Earlier, President Sisi said: “Egypt reserves the right to respond at the proper time and in the appropriate style in retaliation against those inhuman criminal killers.
“Egypt and the whole world are in a fierce battle with extremist groups carrying extremist ideology and sharing the same goals.”
Egypt has declared seven days of national mourning.
Leading international condemnation, the United States called the purported killings “despicable” and “cowardly”.