By his third frantic dash down the stairs, with a wet piece of cloth over his mouth and a little girl in each arm, everything went dark for Khaled Abu Jaafar."I lost consciousness. I couldn't breathe any more; it was like my lungs were shutting down," recalled the resident of Douma, in Syria's Eastern Ghouta."I woke up about 30 minutes later and they had undressed me and were washing my body with water," Abu Jaafar told Al Jazeera on Sunday. "They were trying to make me vomit as my mouth was emitting a yellow substance."Abu Jaafar is one of the survivors struggling to cope with the effects of a chemical attack on Saturday in the besieged town of Douma, the last rebel stronghold near the Syrian capital, Damascus.Rescue workers and medical staff have said at least 85 people were killed in the chlorine gas attack - an accusation dismissed by the Syrian government as "farcical".Among those killed, witnesses said, were many women and children who had sought refuge in the basements of buildings to escape heavy bombardment by pro-government forces. Abu Jaafar, a radio station worker, said that as panicked residents started running around after the attack, he rushed to one of these hideouts to check on his friends and help get people out."While people were in the shelters, some on the roof managed to see the gas bombs as they dropped from the planes," Abu Jaafar said, describing what he said was green gas emanating from the canisters falling from the sky."Those who saw them rushed to tell everyone in the basement to evacuate," he added. "I went up and down the stairs about three times to help evacuate children from the building."Evacuation dealsThe attack came on the second day of a fierce ground and air push by pro-government forces after a period of relative calm. The Syrian army said the offensive was in response to deadly shelling by Jaish al-Islam, the last remaining opposition group in Eastern Ghouta, on residential areas in Damascus. Jaish al-Islam denied the allegation.The group is currently in negotiations with the Russian army, a major ally of Syrian President Bashar al-Assad, over a possible evacuation deal, according to reports carried by state media and pro-Syrian opposition Orient TV.Last week, two other rebel groups reached evacuation agreements with the Russians, which resulted in about 19,000 people leaving for the northern province of Idlib.They included fighters from the Faylaq al-Rahman and Ahrar al-Sham groups, their relatives and other locals.Rebel groups argued that the evacuation amounts to forced displacement, but gave in after weeks of intense bombardment.Meanwhile, remaining civilians continue to endure a bombing campaign and the effects of a crippling government siege that has been in place since 2013.'Unbearable scenes'The chemical attack in Douma is the largest of its kind in Syria since April last year, when nerve agent sarin or a sarin-like substance was dropped onto the town of Khan Sheikhoun, killing at least 85 people.Symptoms of a chlorine attack include dyspnea and coughing, as well as intensive irritation of the mucous membrane and breathing difficulties.On Saturday evening, rescue workers posted videos on social media of people appearing to show symptoms consistent with a gas attack. Some appeared to have white foam around their mouths and noses.Abu Jaafar said that those who did not manage to evacuate their shelters died instantly."There were basements in other buildings with people who didn't see the gas in time. We entered those buildings and found bodies on the staircases and on the floor - they died while attempting to exit," he said.Although some Douma residents rushed to various medical points, a shortage of supplies and doctors meant that treatment options were limited.Activists said that several of Douma's clinics and ambulance teams had been hit during the bombardment campaign, largely disrupting the town's medical assistance capacity.Local activist Alaa Abu Yasser was also among those who tried to help evacuate people."I went to a building where about 35 people had died as a result of this attack; the scenes I saw were unbearable, it's like nothing I have ever seen even in the movies," he told Al Jazeera, describing the aftermath of the attack."As I approached the building, a father was crying hysterically as he dragged his feet towards us carrying his two children … he was hugging them, smelling and kissing them after they suffocated to death," Abu Yasser added.Several witnesses speaking to Al Jazeera said that during a chemical attack it is common practice for people to rush to the top floors and on the roofs of buildings in a bid to avoid inhaling the gas that tends to "stick to the ground"."When we arrived to the roof of the building I was helping at, I saw the lifeless bodies of a mother in her 50s, with two of her adult daughters and a child with their arms around each other, all foaming at the mouth," said Abu Yasser."I mostly saw bodies of women and children in three separate rooms; they've been placed there to isolate the smell of the gas from those who survived," he added.Although the White Helmets, a group of rescuers operating in opposition-held areas in Syria, and Syrian American Medical Society have given a death toll of at least 85, there are fears that the number of people killed in the attack could be higher."The rescue teams have not been able to document all the cases," local activist Mansour Abu al-Khair told Al Jazeera. "They're overwhelmed and cannot deal with the impact of the attack."He explained many of those who lost their lives were still under destroyed buildings and have not yet been pulled from the rubble."Others are instantly being buried by their families, so they aren't accounted for in terms of registered numbers," al-Khair said."We expect the death toll to surpass 100," he added.