Kassem Eid describes it as "Judgement Day".

"We hear about how people will be confused and terrified on Judgement Day," he told the ABC.

"And that's the only way I can describe it: watching women, children and men suffocating, falling on the ground without seeing a single drop of blood."

Children who survived the attack rest inside a mosque. ( Reuters: Mohamed Abdullah )

It was early morning on August 21, 2013 when bombs loaded with a nerve agent fell on the Syrian capital of Damascus.

In the end, 1,429 people —including 426 children — were killed, according to figures from the US Government.

Since then there have been multiple reports of gas attacks in Syria.

In his book My Country: A Syrian Memoir, Kassem Eid describes the moment it began at 4.45am.

My eyes were burning, my head was throbbing, and my throat was rasping for air. I was suffocating. I tried my best to inhale — once, twice, three times. All I heard was that same horrible scraping sound as my throat blocked. The drumming pain in my head became unbearable. The world began to blur. Suddenly my windpipe opened again. The air ripped through my throat and pierced my lungs. Invisible needles stabbed my eyes. A searing pain clawed at my stomach. I doubled over and shouted to my roommates, "Wake up! It's a chemical attack!" Abu Abdo, my high school writing partner; Ahmad, a friend from middle school; and Alm Dar, a Free Syrian Army field commander, scrambled out of their beds in panic. I rushed to the bathroom and slapped water all over my face. I heard a din outside — screams from my neighbours. My friends were also fighting for breath and coughing with all their force. We staggered around the room, panting and retching as we tried to put on our clothes as quickly as possible. Even before we could finish, we heard rapid and urgent bangs at the door. Ahmad ran to open it. It was our neighbour Um Khaled. "Help, please, they're dying," she gasped. She was carrying her children, four and six, one under each arm. Both were unconscious. Their faces were blue and yellow and they were vomiting an ugly white froth from their mouths. Alm Dar ran downstairs to get his old white truck. Ahmad and Abu Abdo picked up the children and followed. I raced through the building to make sure no one else was hurt. I hurried downstairs to the street, rushing past blasted-out windows, crumbling walls, pockmarked floors and piles of rubble. When I reached the front door and looked outside, I stopped short and stared in terror. Dozens of men, women and children were writhing in pain on the ground. Other people were shouting for doctors, praying and calling to Allah in the heavens, pleading for their fallen loved ones to start breathing again. Out of the corner of my eye, I noticed a large lump lying in the dirt about 50 metres to my left. As I moved closer, I realised that it was a small boy with his face to the ground. I ran to turn him over. The sight of his face made me forget every horror I had seen in the past three years: the burned and rotting corpses after massacres, the woman and children shredded into pieces by shelling, the cries of my friends as they lay wounded from combat — I forgot them all. All I could focus on was the innocent face of this boy stained with grotesque shades of red, yellow and blue. His eyes returned an empty, glassy stare. White vomit oozed from his mouth, and a grating sound rasped from his throat as he struggled to breathe. I took off his shirt and tried to blow air into his mouth. I pressed his chest and tried to pump the white poison from his lungs. I screamed for help, begged Allah for mercy. None of it helped. After two or three minutes Alm Dar pulled up in a truck overflowing with injured women and children. He stared blankly at the boy, turned to his overflowing truck, turned back to me. I sat in the back with the boy. He was still struggling to breathe, that horrible grating sound still coming from his throat. We drove past more bodies and wailing survivors. I held him and cried. When we pulled into the field hospital, I lifted the boy down. He seemed heavier than before. I could barely keep my balance and had to use all my strength to lay him on the ground. Then the world began to shimmer and turn grey, and the ground rose up to meet me.

Civilians run after an air raid in Damascus in 2018. ( Reuters: Bassam Khabieh )

From activist to rebel

The bombing marked a drastic escalation in the bloody civil war in Syria and for people like Mr Eid.

The US, UK and France said a 2013 UN report vindicated their stance that the Syrian Government was to blame, while Syrian President Bashar al-Assad denied he was responsible.

Mr Eid has no doubts.

"The Government — our own Government — put us all under siege," Mr Eid said.

"Most rebel towns, they cut off power, food, medicine, and they literally kept bombing us with everything they got.

"I just couldn't believe that someone can be that cruel, to gas children, to gas women."

Just hours after the attack, Mr Eid picked up a gun for the first time to join the Free Syrian Army's fight against government forces.

"It was the only option … either to pick up a gun and defend those civilians or get slaughtered," he said.

The city of Homs has been one of the hardest hit in Syria. ( Reuters: Yazan Homsy )

'Islamic State 2.0'

Mr Eid is furious at the international reaction to the 2013 attack, and the subsequent assaults that continue to rain down on Eastern Ghoutta.

The bandages, aid and food were "very, very kind", he said, but he questions why more wasn't done to remove Mr Assad from power.

It was in this chaos that Islamic State grew, Mr Eid asserts, rallying desperate people to their cause.

Mr Eid said the Syrian people are waiting for Islamic State to rise again. ( Reuters: Yaser al-Khodor )

"The international inaction … fuelled those extremists to come and tell people, 'Look, the world doesn't care about you. We're here to help you'," he said.

"That was the mistake that I really hope the world will acknowledge and try to fix before it's too late."

Just this week there has been an exodus of IS fighters and their families from the extremist group's final enclave in Baghouz.

Sorry, this audio has expired Thousands of civilians flee final IS stronghold in Syria

At least 500 fighters gave themselves up rather than fight to the end after days of bombardment by the Syrian Democratic Forces — a US-backed and Kurdish-led Syrian force.

Yet Mr Eid said he wasn't convinced IS was destroyed.

"We are, in my opinion, waiting for ISIS 2.0 to be created, especially now with talking about the US withdrawal from Syria," he said.

"I still have friends in Syria. I still have activists on the ground who are still telling me that ISIS still exist.

"They have a lot of sleeping cells. They're just waiting for things to settle down, to reboost themselves all over again."