Egypt's chief prosecutor has referred 555 terror suspects to a military court on charges of joining a local affiliate of the Islamic State group.

Monday's statement by the prosecutor says the suspects formed dozens of small militant cells and carried out 63 attacks against army and police forces in the restive northern part of the Sinai Peninsula.

It says only 216 of the 555 suspects are in custody.

Egypt has been battling Islamic militants for years, but the insurgency gathered steam and grew deadlier after the 2013 ouster by the military of Mohammed Morsi, an Islamist president whose year in office proved divisive.

In February, authorities launched a wide-scale operation in Sinai, the Nile Delta and the Western Desert to target "terrorist and criminal elements and organizations."