Egypt's deadliest militant group, Ansar Beit al-Maqdis, pledged its allegiance to the Islamic State organisation in Iraq and Syria, in a recording posted on its Twitter account on Monday.

The announcement is the most significant pledge of support for IS in the region outside Iraq and Syria, suggesting its influence over militant groups is overshadowing its once dominant Al-Qaeda rivals.

Ansar Beit al-Maqdis has waged an insurgency from its Sinai Peninsula stronghold that has killed scores of policemen and soldiers since the Egyptian army ousted Islamist president Mohamed Morsi in July last year.

It was not immediately clear whether Ansar Beit al-Maqdis's pledge would mean a shift in tactics to attack Western targets in Egypt, as demanded by IS.

The group has so far focused its attacks on security forces, and once bombed a tourist bus on the border with Israel, killing three South Koreans.

"We announce our pledge of allegiance to the caliph Ibrahim Ibn Awad... to listen and obey," the audio recording said, referring to IS leader Abu Bakr al-Baghdadi.

"We call on all Muslims everywhere to pledge allegiance to the caliph and support him," the recording said.

The group had previously expressed support for IS but stopped short of pledging its allegiance before Monday, even denying it last week.

Addressing Egyptians, the group demanded they take up arms against President Abdel Fattah al-Sisi, the former army chief who toppled Morsi and unleashed a crackdown on Islamists.

"What are you waiting for after your honour has been violated and your blood spilled... by this stupid tyrant and his soldiers," it said.

It also made a thinly veiled appeal to Morsi's Islamist supporters, who have held regular protests often quashed by police.

"Humiliating non-violence will not be of use to you, nor heretical democracy, and you have seen what happened to its adherents," it said.

Interior ministry spokesman Hany Abdel Latif told AFP the announcement would make no difference to Egypt's fight against the militants.

"They are just different names for the same terrorists," he said.

Ansar Beit al-Maqdis, which means Partisans of Jerusalem, was formed during the security vacuum that followed the overthrow of longtime president Hosni Mubarak in the Arab Spring revolution of 2011.

The group has conducted cross-border attacks against neighbouring Israel, as well as attacks on Egyptian soldiers and police.

It has claimed some of the deadliest attacks on police and soldiers and tried to assassinate the interior minister with a car bomb last year. He survived unscathed.

Several of the group's members who conducted attacks -- including the interior minister's would-be assassin -- fought in Syria alongside jihadists before returning to Egypt to join the group.

It has said it wants to implement Islamic law and avenge Islamists killed in the crackdown that followed Morsi's overthrow.