International community keeps turning blind eye to the crimes being perpetrated in Syria, chemical weapons expert says.

On April 4, 2017, Syrian air raids pummeled the small rebel-held town of Khan Sheikhoun in the northern province of Idlib.

As news of the attack spread, it quickly became apparent that something other than conventional weapons had been used.

Residents told Al Jazeera they saw people choking and foaming at the mouth, “suffocating while their lungs collapsed”.

More than 80 people, including many women and children, died when nerve agent sarin or a sarin-like substance was dropped onto Khan Sheikhoun. Hundreds more were wounded, in one of the worst chemical attacks since sarin gas killed hundreds of civilians in Ghouta, near the capital, Damascus, in August 2013.

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Syria’s government has denied involvement and claims it no longer possesses chemical weapons after a 2013 agreement under which it pledged to hand over thousands of tonnes of deadly chemical agents following US threats of a unilateral attack.

But in October 2017, the Joint Investigative Mechanism, a group tasked by the UN Security Council and the Organisation for the Prohibition of Chemical Weapons (OPCW) to investigate chemical weapons attacks, confirmed that the chemical attack on Khan Sheikhoun did happen, and that the Syrian government was responsible for the killing of civilians.

On the one-year anniversary of the Khan Sheikhoun attack, Al Jazeera’s Faisal Edroos spoke with Hamish de Bretton-Gordon, a chemical weapons adviser to NGOs working in Syria and Iraq, about the usage of chemical weapons by the Syrian government, and what can be done to prevent their use.

Al Jazeera: How many chemical weapons attacks have there been since the start of the Syrian uprising in 2011?

Hamish de Bretton-Gordon: While it’s difficult to gauge exactly what has been done and where, we’ve seen the use of sarin in Khan Sheikhoun as well as in Ghouta – that’s where hundreds of people, including scores of women and children, were killed.

But it’s predominantly chlorine barrel bombs, dropped from regime helicopters, that make up the bulk of the regime’s attacks.

Al Jazeera: What has been the international community’s reaction?

De Bretton-Gordon: It’s ridiculous that the international community continues to turn a blind eye to the crimes being perpetrated in Syria.

A red line has been crossed and the US hasn’t acted as it said it would.

After the Khan Sheikhoun attack, the Trump administration launched a few Tomahawk missiles to take out the Syrian aircraft that bombed the town. But aside from that, they’ve done very little.

Instead, the 100-year taboo on the use of chemical weapons has been completely broken.

Now, any despot, dictator, rogue state, terrorist, sees chemical weapons as a viable weapon. We could end up seeing more chemical weapons proliferation across the world than we’ve been witness to.

Al Jazeera: Why is it difficult to determine whether a chemical attack has taken place?

De Bretton-Gordon: The OPCW can’t get into Syria, and since most attacks happen in rebel-held areas it’s difficult to obtain the necessary evidence.

Assad and the Russians [Syrian government allies] say they don’t use chemical weapons, but the OPCW hasn’t been able to collect enough evidence that can prove otherwise.

The OPCW needs to review of its operations and the international community needs to state categorically that they won’t stand for chemical weapons attacks and will take action – which is something they haven’t been able to do.

Al Jazeera: Most of the information obtained from rebel-held areas is filmed by local activists and subsequently posted on social media. Can videos of chemical attacks uploaded on such platforms be trusted and how do we know they’re not fake news and disinformation?

De Bretton-Gordon: The Union of Syrian Medical Care and Relief Organizations (UOSSM) has more than 50 hospitals across Syria, and we’ve treated many of the casualties affected by the regime’s chemical weapons attacks.

Hospital staff have told me [about treating chemical attacks victims] and even I … investigated a chemical weapons attack.

Hence, I can state quite unequivocally that there have been several chemical weapons attacks in the country.

So with all the evidence I’ve collected over the past four-and-half years, coupled with videos uploaded to social media, eyewitness testimony and samples that have been analysed, we have proof that the Syrian regime has and does use chemical weapons.

Al Jazeera: Pro-Assad supporters have accused some of the victims of being “paid actors” and peddling a false narrative. Is there any truth to this?

De Bretton-Gordon: When people come to our hospitals, they’re received by trained medical professionals, so we can tell if people are faking it. So, no, we haven’t seen any paid actors.

I’m absolutely confident that the Syrians who appear in videos exhibiting trauma from chemical attacks are real and they are who they say they are.

Al Jazeera: Do the rebels have the capabilities to launch chemical attacks?

De Bretton-Gordon: The so-called Islamic State group (ISIL, also known as ISIS) launched chemical attacks against civilian populations in both Syria and Iraq, mainly against the Peshmerga (Kurdish fighters), and its quite possible that al-Qaeda linked groups such as Jabhat Fateh al-Sham (formerly known as the al-Nusra Front) have as well.

But when it comes to groups such as the Free Syrian Army, there hasn’t been any evidence of that.

Al Jazeera: Is there anything that the UN, regional powers can do to stop attacks on civilian areas?

De Bretton-Gordon: The international community has to reimpose its view that the use of chemical weapons is prohibited and it will act firmly should the Assad regime persist.

What has to happen is they have to launch air strikes through the UN Security Council, just as they did after the Khan Sheikhoun attack.

That is the only way we can stop chemical attacks in the future.