Israel has charged seven people with aiding terrorism, belonging to an illegal group and contacting foreign agents.
The defendants, all Arab citizens, and including a lawyer, have denied the accusations.
Israel claims the accused are part of an ISIL cell operating on its territory.
Prosecutors allege the group was planning attacks in Israel and communicated with rebels in Syria.
Experts say dozens of Israeli Arabs and Palestinians have travelled to Syria and Iraq to join insurgent groups.
The seven Israeli Arabs are from the Galilee region and were arrested in November and December.
Details of the case are only now emerging, with reporting restrictions having just been eased.
One of the defendants reportedly tried to leave Israel in July for Syria but was stopped at the airport in Tel Aviv.
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