While Egypt's security forces have been battling an Islamist militants in northern Sinai for years, but the violence picked up after the 2013 ouster of President Mohammed Morsi.
They have also targeted followers of the mystical Sufi branch of Sunni Islam as well as Christians.
Eyewitnesses reported ambulances ferrying casualties from the scene to nearby hospitals after the attack on Al Rawdah mosque in Bir Al Abed, west of Arish city.
The jihadists have also attacked local tribes and their militias, branding them traitors for working with the army and police.
Al-Sisi said that the Friday's attack aimed to spread a seed of doubt among Egyptians about the capacity of the state, but affirmed that the armed forces and police will be responding to restore security in north Sinai. Locals took up weapons to help thwart the attackers, the official said.
At least 155 people have been killed today when suspected militants set off a bomb and opened fire at a mosque in Egypt.
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"Will be calling the President of Egypt in a short while to discuss the tragic terrorist attack, with so much loss of life".
View of the Rawda mosque after a gun and bombing attack on November 24, 2017.
Leaders around the world condemned the mosque attack.
The Sinai branch is one of Islamic State's surviving branches following the collapse of its self-declared caliphate in Syria and Iraq after military defeats by US -backed forces.
UK Prime Minister Theresa May and French President Emmanuel Macron also gave their condolences on Twitter, while NATO Secretary General Jens Stoltenberg said his thoughts were with all those affected. Condolences with all those in #Egypt affected by this evil and cowardly act.
"What is happening is an attempt to stop us from our efforts in the fight against terrorism, to destroy our efforts to stop the awful criminal plan that aims to destroy what is left of our region".