Egyptian forces have launched a strike against “terrorist training camps” in retaliation for an attack on Coptic Christians.
Gunmen earlier attacked a bus carrying worshippers in central Egypt, killing at least 28 people and wounding 25 others, officials say.
President Abdul Fattah al-Sisi said he “will not hesitate to strike terrorist camps anywhere”.
It is not clear whether the attack was within Egypt’s borders.
In a televised speech, President Sisi said: “We will not hesitate to protect our people from the evil.”
He said that countries which support terrorism should be punished, and 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.