Israel charged a member of its Arab minority on Wednesday with security offences for joining the Syrian insurgency against President Bashar al-Assad, saying he had associated with radical Islamist fighters.
An Israeli official said Hikmat Massarwa's case was the first of its kind. Indicted for unlawful military training, travel to a hostile country and contacts with foreign agents, he could be jailed for up to 15 years if convicted.
Massarwa, 29, was arrested on March 19 upon returning from Syria, where he helped set up a rebel base and underwent weapons training, Israel's Shin Bet domestic intelligence service said in a statement.
Describing Massarwa's fellow rebels as members of the "Global Jihad" - an Israeli term for al Qaeda and allied groups - the Shin Bet said Massarwa had been questioned while in Syria about Israel's military and its Dimona nuclear reactor.
Through his lawyer, Massarwa acknowledged going to Syria to join the rebels, but denied that they were jihadis or that his actions posed any danger to Israel.
"He spent all of one week in Syria, and that was in the company of mainstream Syrian rebels fighting Assad," the lawyer, Helal Jaber, told Reuters. "The claim that he sought to harm (Israeli) national security is baseless."
Jaber said his client had no links to militants or criminal groups in Israel or abroad, and that he worked as a baker in the Israeli coastal town of Herzliya.
The Israeli official, speaking on condition of anonymity, said Israel knew of "a few other" of its citizens who had volunteered for the Syrian insurgency and were "still out there, in the field".
Arabs, most of them Muslim, make up around 20 percent of Israel's population. They seldom take up arms with its enemies.
Distinguishing between mainstream rebels who might help stabilise Damascus, should Assad fall, and foreign-born volunteers who see the insurgency as part of a wider jihad has bedevilled world powers as they try to craft Syria strategy.
Israel, which enjoyed a decades-long, stable stand-off with Syria under the largely secular rule of the Assad family, believes that around one in 10 of the rebels are Sunni Muslim radicals.
The insurgency has drawn close to the Israeli-occupied Golan Heights - taken from Syria in a 1967 war - in recent months, stirring fears of spillover violence and retaliation by Israel.
The Shin Bet statement said another reason Massarwa went to Syria was to track a brother who had travelled there several months earlier to join the rebels. Jaber confirmed that Massarwa had a brother unaccounted for in Syria.
Massarwa entered and left Syria through Turkey and was arrested after landing at Tel Aviv airport, Jaber said, adding that what might have tipped off the Israelis was the defendant's request that his family wire him $700 for the flight home.
The Shin Bet said Massarwa's fellow rebels tried to talk him into carrying out a suicide attack against Assad forces, or a similar strike in Israel, but that he refused.
Jaber denied such requests were ever made of his client. He played down the significance of Massarwa's discussions with the rebels about Israel's military capabilities, saying anything he said would not have gone beyond information publicly available.