An Israeli expert on chemical weapons believes that forces loyal to Syrian President Bashar Assad used sarin gas in an attack on Idlib province on Tuesday . Dr. Dany Shoham, an expert on biological and chemical weapons, is also skeptical of the Russian claim that the regime struck a weapons depot which the rebels had been using to store poison gas.

Shoham, a researcher at Bar-Ilan University's Center for Strategic Studies, says there is a high probability that the agent that caused dozens of deaths in the town of Khan Sheikhoun was sarin nerve gas, though he concedes that it cannot be identified from photographs alone. Only chemical analysis can categorically identify the chemical, he says, but “the pictures are highly reminiscent of the event in 2013, when Assad’s army definitely used sarin and about 1,400 people, including children, died because of it. The pictures are really similar.”

The World Health Organization said on Wednesday that the victims appeared to show symptoms consistent with reaction to a nerve agent. "Some cases appear to show additional signs consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents," the WHO said in a statement, putting the death toll at at least 70.

Regarding Russian claims that chemicals were been released when an airstrike hit a weapons warehouses belonging to the anti-regime rebels, Shoham says it highly unlikely that the rebels were in possession of nerve gas.

Open gallery view A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4 Credit: AMMAR ABDULLAH/REUTERS

Shoham says that while the rebels have also used chemical weapons in the past – mainly chlorine, but also mustard gas – and may have those agents, in principle the likelihood that they have nerve gas is very low.

“Throughout, the Russians have been backing the Syrians, covering up and masking the many events in which chemical weapons have been used by the Syrians," says Shoham. “So, in this case too, clearly that is what they’re trying to do. They denied use by Assad’s army in the past so the chance is small that this time the Russian claim is true.”

Answering arguments that an explosion resulting from a strike would have neutralized the sarin, Saham points out that indeed it could, but not all the gas – some would have “escaped” and retained efficacy. “In that scenario, some sarin would escape to the environment I would not cling to that argument because in any case, some sarin would be released, and would be effective,” he says.

Dr. Emily Landau, head of the Arms Control and Regional Security Program and the Institute for National Security Studies, said Wednesday that “in Syria there is always use being made of different types of chemical weapons, but those that are not defined as such under the convention. There was use of chlorine, which generally doesn’t kill, and now there was a step up with the use of sarin.”

Landau added that the use of chemical weapons sows fear and terror among the people, and can help the Syrian regime convey a message to them to evacuate the area. “There’s no one to stop this. We see the Russian response, even though all the assessments speak of Assad, and we see regimes that have no problem agreeing to something and then doing whatever they want."

According to Israeli security officials, reports that Assad was responsible for the chemical attack are highly probable. Sources in Israel said that the assault was approved by the highest levels of the Syrian regime, but stopped short of saying whether Assad’s patrons, Russia and Iran, were involved.

While the Syrian army has published a statement denying any use of chemical weapons, Israel has discounted the regime’s denial.

Reuters contributed to this report.