Egyptian forces have struck “terrorist training camps” in retaliation for an attack on Coptic Christians, President Abdul Fattah al-Sisi says.
State media say six strikes hit the town of Derna in neighbouring Libya.
Gunmen earlier attacked a bus carrying worshippers in central Egypt, killing at least 28 people and wounding 25 others.
President Sisi said he would “not hesitate to strike terrorist camps anywhere”.
Announcing the strike in a TV speech late on Friday, he promised to “protect our people from the evil”.
Countries which supported terrorism should be punished, he said, as he appealed to US President Donald Trump for help.
Although no group immediately said it was behind the attack, Islamic State (IS) militants have targeted Copts several times in recent months, and vowed to do so again.
“Daesh strategy depends on driving a wedge between Muslims and Christians,” Mr Sisi said, using an alternative name for IS.
The Copts killed on Friday had been travelling to the Monastery of St Samuel the Confessor, 135km (85 miles) south of Cairo, from Minya province when their bus came under fire.
It was in a small convoy that was stopped on a desert road near a police station on the border with Beni Suef provinces.
Up to 10 gunmen wearing military uniforms attacked the convoy, eyewitnesses said. The gunmen then fired at the vehicles with automatic weapons before fleeing, they added.
Minya Province Bishop Makarios said many of the victims were shot at point blank range, the New York Times reported.
He said that children had been on the bus and were among the dead, adding that a pick-up truck in the convoy carrying workmen at the monastery was also targeted.
Minutes after President Sisi’s speech, the White House released a statement of support from Mr Trump.
“This merciless slaughter of Christians in Egypt tears at our hearts and grieves our souls,” he said.
“America stands with President Sisi and all the Egyptian people today, and always, as we fight to defeat this common enemy,” he added.