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Egypt's governor for North Sinai announced on Friday the closure of all schools "indefinitely" as the military presses forward with a crackdown on militants.

A major offensive on militants in Sinai, Nile Delta and the Western Desert region began earlier in the day as the Egyptian military announced in a televised address that a push was being launched against "terrorist" strongholds in the country, after years of militant activity.

Armoured vehicles and tanks were reported heading north in the Sinai, while hospitals were placed on high alert to prepare for possible casualties.

Doctors and medical staff were notified on Wednesday that all staff vacations had been cancelled for the next six months in anticipation of the assault, Mada Masr reported.

Associated Press reported that gas stations and shops were also ordered shut.

Egypt has been battling a growing militant insurgency in the Sinai Peninsula since the overthrow of the elected former president Mohamed Morsi in 2013.

In recent months, dozens have been killed in gunfire exchange between Egyptian security forces and suspected militants.Egypt's Interior Ministry announced that the national alert level had been raised to "maximum security" in all governorates.

Palestinian media outlets also reported the closure of the Rafah crossing connecting Sinai to Gaza ahead of the operation, following a sudden decision by Egypt to temporarily re-open the crossing on Wednesday.

Last month, militants in northern Sinai killed 305 people in the deadliest assault by militants in Egypt's modern history, at a mosque near al-Arish.

After the mosque attack, President Abdel Fattah al-Sisi instructed his military chief of staff to quell the attacks in three months using "complete brutal force".

"In three months, with God's grace, and with your help and sacrifices and those of the police, Egypt will restore stability and security in Sinai," Sisi said.

"Complete brutal force will be used. Complete brutal force."

The Sinai Peninsula had long been demilitarised under the terms of the 1979 peace treaty between Egypt and Israel, but as the violence intensified the government responded by ramping up its military presence, with the tacit approval of Israel.

A report in the New York Times said that for the past two years Egypt has been in a secret alliance with Israel, allowing Israeli drones to fly over the northern Sinai Peninsula and conducting more than 100 covert airstrikes, but the claims have been strongly denied by Cairo.