General Prosecutor Nabil Sadek said in a statement that another 128 people were wounded in the attack, when heavily- armed militants bombed the al-Rowda mosque in Al-Arish city and opened fire on people attending the Friday prayers.
The death toll was reported at 235 yesterday.
Sadek said the number of terrorists involved in the attack varied from 25 to 30, some of whom had raised the Islamic State flag during the attack.
The Egypt government announced three days of mourning, even as President Abdel Fattah el-Sisi chaired an emergency meeting with officials to review security after the attack.
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Egypt's Army said in a statement that a number of terrorists have been killed in air strikes in North Sinai since the attack yesterday. The Army was conducting raids in the troubled region to eliminate terrorist hideouts.
The Al-Ahram newspaper, citing a military source, reported that a military operation was underway in North Sinai. The unnamed source did not reveal the details of the operation, but told the daily: "The response will be on the ground and will not stop until the elimination of everyone involved in the attack. We are taking our revenge now."
During the attack, the terrorists took positions near the door and the 12 windows of the mosque and then opened fire on the worshippers, the statement said.
They had come to the mosque in five SUVs and torched another seven vehicles parked outside the mosque and owned by the worshippers there, the prosecutor said.
According to people injured in the attack, some of the terrorists had masked their face and all of them wore military-like dresses. After the bomb ripped through the mosque, the gunmen on four off-road vehicles opened fire on worshippers who tried to escape from the site after the blast.
Speaking to state-run Masriya TV, Egypt's health ministry spokesman Khalid Mujahid described it as a "terrorist attack."
One report said the target appeared to be the supporters of the security forces who were praying at the mosque. Local residents were quoted in news reports saying that followers of Sufism, or Islamic mysticism, often gathered at the mosque.
Islamist jihadist groups, including the Islamic State, see Sufis as heretics.
The Sinai peninsula has endured many attacks blamed on the Islamic State and other terrorist groups since the January 2011 revolution that toppled president Hosni Mubarak, but this was the deadliest assault of its kind.
In May this year, gunmen attacked a bus carrying Coptic Christians in central Egypt, killing at least 28 people.
In April, two suicide bombings at Palm Sunday service at churches in the northern cities of Alexandria and Tanta killed 46 people.