At least 235 people were killed as militants bombed a mosque in Egypt's north Sinai region on Friday.
He said the terrorists killed innocent people, did not distinguish between children, youth or elders, and the attack was a war crime.
Hours after the attack, Egypt's military launched air strikes on targets in mountainous areas around Bir al-Abed, security sources and witnesses said.
Egypt's presidency declared a three-day mourning period, as President Abdel-Fattah el-Sissi convened a high-level meeting of security officials.
Meanwhile, Vice President Mike Pence offered his condolences to victims of the attack, many of whom are followers of Sufism, a mystical branch of Sunni Islam that many jihadist groups view as heretical.
Images from inside the building showed dozens of bodies wrapped in blood-soaked cloth lined up on the carpeted floor.
It is the first time a mosque is targeted by militants in Egypt.
The group had, since that attack, swore loyalty to ISIS and is now known as the Islamic State of Iraq and the Levant - Sinai Province [ISIL-SP].
Appalled by the sickening attack on a mosque in North Sinai.
In a statement afterward, el-Sissi said the attack "will not go unpunished" and that Egypt will persevere with its war on terrorism.
The president further promised that Egypt's army and policemen will shortly reestablish control and security, asserting, "we will respond to this terrorist attack vigorously". "The world can not tolerate terrorism, we must defeat them militarily and discredit the extremist ideology that forms the basis of their existence!" the POTUS tweeted.
President of Evangelical Church in Egypt Andrea Zaki condemned the terrorist attack, saying it aims at destabilizing the security and stability in Egypt.
Egyptian security forces face nearly daily attacks from ISIS-aligned militants, whom they have been battling in northern Sinai for years.
The president said that Egypt will stand strong in the face of terrorism, and will continue its war against it.