BEIRUT: ISIS has again used chemical weapons in Syria, and may have up to 200 tons of mustard gas, experts have said. Elsewhere in Syria Monday, the regime continued to pound the besieged Eastern Ghouta area outside of Damascus, where the death toll from 24 hours of attacks reached 31, the Syrian Observatory for Human Rights said, and five civilians were killed in a regime-held area of Aleppo, state TV reported.

Around 50 civilians exhibited symptoms of exposure to mustard gas north of Aleppo last Friday, the Syrian American Medical Society said, with local sources reporting to them that ISIS had launched 50 shells in the attack.

The chemical weapon, developed in World War I, can be fatal, but no deaths have been reported yet from this latest attack. Photos show the victims suffering from skin blisters and lesions, and many experience respiratory irritation, wheezing, coughing, irritation and redness of the eyes and mucous membranes, skin irritation and severe itching, SAMS said.

But unlike chorine, which has a half-life of a few minutes, mustard gas is a persistent agent, Hamish de Bretton Gordon, a chemical and biological weapons expert told The Daily Star, meaning that it can remain in the buildings and in the ground for months or even years.

De Bretton Gordon cautioned against civilians returning to the area. “One of the really big issues is that I understand people are returning to the scene and becoming ill,” he said.

Mustard gas, he said, is typically used in warfare to “deny ground,” so that people are unable to return to the area, or to the buildings, hit.

Aside from the initial symptoms, when the victim has been in direct contact with the gas – which in this form was liquid, but can also be vaporized – it can also be carcinogenic and mutagenic, De Bretton Gordon said.

“It was designed to incapacitate people,” he said, as “having injured people is more effective than having dead people,” as it requires more energy to look after them. “It is not designed to kill a lot of people quickly, but it is also carcinogenic and mutagenic,” so some 27 years after the Halabja chemical massacre northern Iraq in which mustard was one of the gases used, people are still dying there.

He said that it was likely that ISIS carried out this recent attack in Marea, as they were known to have committed a similar attack recently, and that they could have a “lot more” mustard gas in reserve, a weapon which he called the “ultimate terror weapon.”

The U.S. and German governments recently confirmed reports that ISIS had used mustard gas against Kurdish soldiers in Hassakeh, northern Syria.

The mustard gas was likely from reserves previously belonging to the regime in Damascus, de Bretton Gordon said, as when President Bashar Assad agreed to carry out an inventory of stocks, it was widely believed that 200 tons of mustard gas was missing, he added.

SAMS said in a statement that Marea, “on the front line of fighting between ISIS and nonstate armed groups, and the SAMS-supported hospital [in Marea] has witnessed increased levels of civilian injuries and mass displacement in recent months.”

Elsewhere Monday, the death toll in Eastern Ghouta rose, with the Observatory reporting that eight children and seven women were among the 31 dead over the last 24 hours. The regime also dropped barrel bombs in Daraya and elsewhere in Idlib province.

Five civilians were killed when rebels shelled a regime-held area of Aleppo city, state TV reported, and 15 were injured in similar rebel attacks on Damascus city.

The regime also bombed the city of Palmyra, the Observatory reported. ISIS took the city from the regime in June.

Over the last month, attacks by ISIS have killed 51 Syrian soldiers in a besieged air base in the northern province of Aleppo, the Observatory also reported Monday. It said regime forces kept up airstrikes against ISIS to stave off the militants, as the extremist group stepped up offensives against both government forces and other insurgents in northern Syria.