The jihadist group's Amaq news agency claimed in a statement that its fighters "set up a checkpoint" for the Christians as they headed to the monastery, shooting at them and setting one of their vehicles on fire.
President Sisi said he would "not hesitate to strike terrorist camps anywhere".
The bus came under fire on a side road in the desert on its way to the remote monastery of St. Samuel the Confessor in Maghagha, about 140 miles south of Cairo.
Egypt's Foreign Minister Sameh Shoukry told his United States counterpart Rex Tillerson that jihadists who attacked a Christian convoy had trained in Libyan militant camps, a statement said on Saturday.
Egyptian media said their country's warplanes targeted an operations room location for the Derna Shura Council in eastern Libya, saying it is backed up by IS terrorists, despite the fact that Derna Shura Council is the only entity that fought IS terror groups and eradicated them from Derna previous year.
There was no immediate claim of responsibility for Friday's attack.
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"Evil organizations of terror" with a "thuggish ideology" are culpable for the attacks against Christians, he said. Coptic Christians, which make up about 10 per cent of Egypt's 91 million population, have faced persecution in Egypt, which has spiked since the toppling of Hosni Mubarak's regime in 2011.
"Wherever innocent blood is spilled, a wound is inflicted upon humanity", Trump said in a statement.
Trump added that the attack "steels our resolve to bring nations together for the righteous objective of crushing the evil organization of terror, and exposing their depraved, twisted and thuggish ideology".
Eight unidentified gunmen were reportedly responsible for yesterday's attack.
Egypt has repeatedly expressed concern over militants crossing from Libya to Egypt to conduct attacks. In December, a suicide bomber targeted a Cairo church.
Then in April, during Palm Sunday services, there were coordinate bombing attacks on Coptic churches in two different cities, killing over 75 people and wounding dozens more.
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