Reports that the regime of Syrian President Bashar Assad was responsible for the deadly chemical attack in Syria on Tuesday are highly probable, according to Israeli security officials.

Sources in Israel said that the assault, which killed at least 100 civilians and injured hundreds more, was approved by the highest levels of the Syrian regime, but at this point it is unclear whether Assad’s patrons, Russia and Iran, were involved.

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Open gallery view Syrian children wait to receive treatment at a makeshift clinic following reported air strikes by government forces in the rebel-held town of Douma, on the eastern outskirts of Damascus, on April 4, 2 Credit: ABD DOUMANY/AFP

The Syrian army published a statement denying any use of chemical weapons. Israel has discounted the regime’s denial, including its claims that a Syrian aircraft struck with conventional weapons, but that during one such strike, a chemical storage tank held by the rebels was hit.

Israeli sources said they believe the assault, which was directed at a village in the rebel-held province of Idlib, was intended to convey a threatening message to insurgent groups that have broken the cease-fire agreement over the past two weeks.

>> Analysis: Assad's chemical attack shows last stage of Syria's war could be most deadly yet >>

It is believed that Assad, or senior officials in his regime, approved the use of chemical weapons against the backdrop of growing confidence in the regime’s stability following military successes since the regime completed its takeover of Aleppo in December.

Israeli security officials said they believe that even after the 2013 agreement to dismantle its chemical weapons stockpiles, the Syrian regime still kept residual quantities of chemical weapons, including sarin nerve gas. Most of the infrastructure for producing these weapons was destroyed as part of the agreement, and it is possible that Syria has been trying to rebuild the CERS weapons plant, including apparently relaunching the manufacture of chemical weapons.

Nevertheless, the gas used on Tuesday is likely to be the remnants of old stockpiles the Syrian regime held onto. Since the agreement, the regime has used chemical weapons of various types on a number of occasions, but this is the first time in almost four years that sarin, a particularly lethal gas, was used.