CAIRO: One of Egypt's most wanted men, a former special forces officer turned Islamist militant commander, has condemned President Abdel Fattah al-Sisi and called for a holy war against his government.

An audio message attributed to Hisham al-Ashmawy, who security officials suspect masterminded the recent car bomb assassination of Egypt's top prosecutor, said the country is "overpowered by the new pharaoh" Sisi.

Ashmawy is part of a small but highly dangerous succession of former Egyptian army officers who have joined militant groups, complicating Sisi's efforts to confront what he calls an existential threat from extremism.

With their knowledge of the Arab world's biggest army and training, they pose a security threat to strategic U.S. ally Egypt which faces an insurgency based in northern Sinai.

"All of you must come together to confront your enemy. Do not fear them, but fear Allah if you are truly believers," said the audio message carried by U.S.-based monitoring group SITE, which it said was posted on July 20 on an al Qaeda-affiliated forum.

The comments coincided with the Eid al-Fitr holiday just marked by Muslims.

The audio message features two pictures of Ashmawy in military uniform. He is identified by the nom de guerre of Abu Omar al-Muhajir al-Masri and as Emir of the Al-Murabiteen group.

Militants have stepped up attacks on soldiers and police since Sisi, as army chief, toppled Islamist President Mohammad Morsi in 2013 after mass protests against his rule. Hundreds have been killed.

Ashmawy, who has been on the run for years, formed a cell within a militant group recently renamed Sinai Province which has pledged allegiance to ISIS, the ultra-hardline Sunni organisation based in Iraq and Syria.

He accused Sisi "and his soldiers" of fighting "our religion" and killing "our men and women".

Sisi has launched a tough security crackdown since Morsi's overthrow. Security forces killed hundreds of supporters of his Muslim Brotherhood in the streets and arrested thousands of others.

Ashmawy is described by security officials as the leader of Sinai Province's military training committee. His cell carried out an operation in October that killed 33 members of the security forces in one of the bloodiest attacks in years.

Egypt closely watches the military for any signs of Islamist-leaning officers, though a senior military official told Reuters defections are not a phenomenon.

A militant cell headed by army officers assassinated President Anwar al-Sadat in 1981.