Egyptian President Abdel Fattah al-Sisi said the country's military and police would respond "with brutal strength" in a televised statement to the nation on Friday after an attack on a mosque in North Sinai left more than 300 people dead.Naming the perpetrators "Takfiris", a term for hardline Sunni Muslims, he announced a period of mourning after the attack and said it would increase the country's resolve to oppose terrorism. "[The military and police] will return security and stability with absolute power, in the coming short period of time. This is our retaliation. We will respond to this attack with brutal strength to encounter these terrorist extremists, the Takfiris," said Sisi.Militants detonated a bomb and gunned down worshipers in the deadliest attack in the country's modern history on Friday, state media and witnesses said. No group immediately claimed responsibility, but since 2013 Egyptian security forces have battled a stubborn Islamic State affiliate in the mainly desert region, and militants have killed hundreds of police and soldiers."We are fighting this alone. Egypt is confronting terrorism on the region and the world's behalf. This is why all that is happening at the moment is an attempt to stop our efforts to confront terrorism and an attempt to destroy our will which aims to stop the criminal scheme which is aiming to destroy what is left of our region," added Sisi."A mourning period has been announced for this painful, traitorous, vile and cowardly attack. It was aimed at destroying our morale. It was aimed at destroying our strength. It was aimed to [increase] doubt [in] our abilities."This painful terrorist attack, will increase our strength, power and will to stand and oppose terrorism."Egypt's military began conducting air strikes around the area of North Sinai where the mosque attack occurred on Friday, security sources and eyewitnesses said.The strikes have been concentrated in several mountainous areas surrounding Al Rawdah mosque where militants are believed to be hiding out, the security sources said.