Conceivably, Israel could covertly support the enemies of Islamic State. Israel has been doing just that in regard to Syria. During the past two years it has launched airstrikes against Assad’s forces which has helped the rebels. Israel takes the position that any attacks on its territory from Syria are the responsibility of the Assad government even if they are made by the rebels.
Moreover, just like the United States, Israel has provided training for Syrian rebels. For example, Abdul-Ilah al-Bashir al-Noeimi, currently the Chief of Staff of the Supreme Military Council (SMC) of the Free Syrian Army, secretly trained in Israel in 2013 after being admitted into the country for medical treatment. [See “Report: Commander of Syrian Rebels Trained in Israel, Jewish Press News Briefs,” Feb. 24, 2014. In regard to Israeli participation in training Syrian rebels, see: Jason Ditz, “Report Claims US, Israeli Trained Rebels Moving Toward Damascus,” Antiwar.com, Aug. 25, 2013,; Jinan Mantash, “Israeli analyst confirms link between Israel, ‘moderate’ Syrian rebels,” Alakbar English, Oct. 17, 2014.]