Egyptian state media say at least 235 people were killed when militants attacked a mosque in north Sinai with explosives and gunfire on November 24.
State media added that more than 100 people were injured in the attack in the town of Bir al-Abd, some 40 kilometers from the provincial capital, El-Arish.
The government has declared three days of mourning.
State television reported that Egyptian President Abdel Fattah al-Sisi convened an emergency security meeting shortly after the attack.
"We will reply to this terrorist act with harsh force against those terrorists," Sisi said.
The Egyptian Air Force reportedly carried out air strikes in the area hours after the attack.
The Associated Press quoted three police officers as saying that assailants in four off-road vehicles detonated explosives before opening gunfire on worshipers inside the mosque during weekly Friday Prayers.
There has been no claim of responsibility.
U.S. President Donald Trump condemned the attack as "horrible and cowardly" and called Sisi to offer condolences.
"The world cannot tolerate terrorism, we must defeat them militarily and discredit the extremist ideology that forms the basis of their existence!" Trump wrote on Twitter.
Egyptian authorities have been battling Islamic State militants in the northern Sinai Peninsula.