Egypt has launched airstrikes on vehicles linked to a bomb and gun attack on worshippers at a mosque that left at least 305 dead and 128 wounded.
The country's chief prosecutor said 27 children were among the dead.
President Abdul Fattah al-Sisi had vowed to respond "with the utmost force" after militants staged the brutal assault during Friday prayers at the al-Rawdah mosque in the northern town of Bir al-Abed.
The mosque was frequented by Sufis, members of a mystic movement within Islam that is viewed by extremists as heretic.
No one has yet claimed responsibility, but it is thought to be the latest - and deadliest - attack by the area's affiliate of the so-called Islamic State group.
Mr Sissi said the terror act "will not go unpunished" in a televised address on Friday.
Hours later, Egypt's military later said its warplanes had destroyed several vehicles which it said helped carry out the attack. It said the strike had killed all passengers inside the vehicles.
Witnesses said at least a dozen attackers had descended on the al-Rawdah mosque in four off-road vehicles before blocking the entrances and slaughtering worshippers.
Officials cited by the state news agency MENA said the attackers fired rocket-propelled grenades and shot men as they tried to run from the building.
The militants blocked off escape routes with burning cars, three police officers on the scene said.
The attackers then escaped, apparently before security forces could confront them.
Pictures broadcast on Egyptian television show scores of ambulances and emergency vehicles arriving at the scene as the wounded were carried out on stretchers.
Egypt's presidency declared a three-day mourning period for the attack, which is the deadliest ever to hit Egypt in the modern era.
Mr al-Sisi convened a high-level meeting of security officials to agree a response, pledging to push forward with the country's war on terror.
"The armed forces and the police will avenge our martyrs and restore security and stability with the utmost force," he said in a statement.
“What is happening is an attempt to stop us from our efforts in the fight against terrorism, to destroy our efforts to stop the terrible criminal plan that aims to destroy what is left of our region.”
He expressed condolences to the victims and their families of the "cowardly" assault.
There was also condemnation from around the world.
Prime Minister Theresa May said she was appalled by the "sickening attack".
US President Donald Trump described the attack as a "horrible and cowardly terrorist attack".
Paris mayor Anne Hidalgo said the Eiffel Tower will go black at midnight in homage to the victims of the deadly assault. She said that turning off the lights at the famed Paris monument would send a message of solidarity from the French capital that has itself been the site of a spate of deadly extremist attacks in recent years.
The attack was the first major militant attack on a Muslim mosque and the the largest single targeting of Egyptian civilians even dating back to previous Islamic militant insurgency in the 1990s.
Insurgents have stepped up a campaign of violence in northern Sinai after the military ousted President Mohammed Morsi from power in 2013 and launched a fierce crackdown on the Muslim Brotherhood group.
Militants have been unable to grab control of territory, but have staged a series of bloody attacks.
Most of the militant actions have targeted military and police, though they have also assassinated individuals the group considers spies for the government or religious heretics.
Members of the Sufi sect have also been attacked. Last year, the militants beheaded a leading local Sufi religious figure, the blind sheikh Suleiman Abu Heraz, and posted photos of the killing online.
Egypt has also faced attacks by militants in its Western Desert.