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.
Libyan officials said Egypt had hit targets in the militant-held city of Derna in co-ordination with Libya.
A video emerged on Sunday showing militants forcing a group of men to the ground and decapitating them.
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 victims were all wearing orange overalls as in previous videos of IS executions. It was one of the first such videos to come from an IS group outside its core territory in Syria and Iraq.
Egypt did not give the locations of the strikes, but a spokesman for Libyan Prime Minister Abdullah al-Thinni told the BBC that Egyptian jets had taken part in co-ordinated air strikes on Derna.
“Eight strikes have been conducted so far (in Derna). The plan is to target all IS locations in the country wherever they are,” said Mohamed Azazza.
Libyan Air force commander Saqer al-Joroushi told Reuters that Libyan planes had bombed targets in Sirte and Bin Jawad.
Earlier, President Abdel Fattah al-Sisi said Egypt had the “right to respond” against IS, whom he described as “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 killings “despicable” and “cowardly”.
Libya is home to a large community of both Muslim and Coptic Egyptians, with most working in the construction sector.
In the first kidnapping in Sirte, in late December, a group of Coptic Christians was abducted at a fake checkpoint while trying to leave the city.
Days later, militants raided a residential compound in Sirte and separated Christians from Muslims before handcuffing their captives and taking them away.
Libya has been in chaos since 2011 and the overthrow of its then-leader, Col Muammar Gaddafi.
Since then, numerous militia groups have battled for control.
It has two rival governments, one based in Tripoli and the other in Tobruk.
Meanwhile, the eastern city of Benghazi – where the 2011 revolution began – is largely in the hands of militant fighters some with links to al-Qaeda.
The head of the US Defense Intelligence Agency warned last month that IS was assembling “a growing international footprint that includes ungoverned and under-governed areas”, including Libya.
On Sunday, Italy closed its embassy in Tripoli.
Italy, the former colonial power, lies less than 500 miles (750km) from Libya at the shortest sea crossing point.