On Friday Morning, a massive explosion in Al-Rawada mosque, located in the town of Beer El Abed, in North Sinai's Al-Arish.
"What is happening in North Sinai is a real reflection of our efforts to combat terrorism, and Egypt is in this war alone on behalf of the region, and the entire world" al-Sisi went on to say. Egypt's presidency has also declared a three-day mourning period, Egyptian state news agency MENA reported.
An attack on a mosque in Egypt has left dozens dead and at least 75 injured.
The BBC reported Egyptian President Abdul Fattah al-Sisi will meet with security forces Friday.
The attack was unusual in several ways for Egypt, said Kamran Bokhari, senior analyst at George Washington University's Program on Extremism: the high casualty toll, targeting a mosque and its level of sophistication.
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.
In October 2015, 224 people were killed when a bomb brought down a Russian passenger jet that crashed in the Sinai Peninsula.
Appalled by the sickening attack on a mosque in North Sinai.
Friday's attack drew swift condemnation from leaders throughout the Middle East and beyond.
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 USA -backed forces.
UK Foreign Secretary Boris Johnson said he was deeply saddened by the "abhorrent attack".
Prime Minister of United Kingdom Theresa May called the attacks "sickening" "evil" and "cowardly". "My honest condolences to all those affected by such a barbaric act", he tweeted.
On the Jordanian royal family's official Twitter account, King Abdullah II condemned what he called a "heinous terrorist attack". Our deep condolences at the loss of innocent lives.
"We stand with Egyptians in their war against terrorism and against anyone who tries to harm the Egyptian national security", Palestinian Authority President Mahmoud Abbas said in a statement.