Egypt's military has set a 48-hour deadline after which it will deal decisively with "violence and terrorism", said a Thursday statement on a military-linked website, on the eve of pro-army rallies.

The statement said military chief Abdel Fattah al-Sisi issued the deadline in a Wednesday speech for all Egyptians "to return to the national fold and prepare for the future".

Sisi had called for mass rallies on Friday to give him a "mandate" to fight what he called "terrorism and violence".

Islamist supporters of ousted president Mohamed Morsi have been accused of using violence to restore the leader overthrown on July 3, although most of their protests are peaceful.

The statement appeared on a unofficial Facebook account with ties to the top council of military generals.

At the expiry of the deadline late on Friday, "the strategy for dealing with violence and terrorism will change...appropriately to deal guarantee security and stability", it said.

A senior army official told AFP that the statement did not reflect the military's point of view, although it appeared on a "page with links to the armed forces".

"The 48-hour ultimatum is a political invitation," the officer said. "It doesn't mean that after 48 hours we are going to crack down."

Sisi's call on Wednesday, in a televised speech, sparked outrage among Islamists and concern in Washington.