The United States is threatening a "strong, swift response" to what it describes as "signs that the Assad regime may be renewing its use of chemical weapons" against rebels in north-west Syria.

Last Friday, Jonathan Cohen, the acting US representative at the United Nations, warned:

"The United States’ resolve to hold the Assad regime accountable for any further use of chemical weapons is unwavering, and we remind the regime and its allies that any use of chemical weapons, including chlorine gas, will be met by a strong, swift response."

The US State Department followed this up on Tuesday, referring to an alleged chlorine attack reported to have taken place on Sunday morning (19 May). 

It said in a statement:

"We are still gathering information on this incident, but we repeat our warning that if the Assad regime uses chemical weapons, the United States and our allies will respond quickly and appropriately."

The specific mention of chlorine gas in Cohen's warning may be significant. Although dozens of chlorine attacks have been reported during the conflict in Syria, international attention has focused mainly on the nerve agent sarin which, though more deadly, has been used less often.

In 2013, when a sarin attack on Ghouta killed hundreds of people President Obama threatened military action but the crisis was defused when Syria agreed to join the Chemical Weapons Convention.

In 2017, after sarin was released in Khan Sheikoun, Donald Trump responded with airstrikes directed against the Assad regime.

In 2018, western powers launched airstrikes after dozens of people were reported killed by a chemical attack on Douma. At the time, there were allegations that sarin had been used but an official investigation by the OPCW later found no evidence of sarin and concluded that the substance involved was probably chlorine.

Despite being less lethal than sarin, chlorine is a very nasty chemical and its use as a weapon is banned under the Chemical Weapons Convention. One effect of its repeated use in Syria is to undermine the international taboo on chemical weapons – so some kind of response is needed in order to uphold the principles established by the Convention.

In terms of any military response, however, chlorine is even more problematic than sarin. 

The first problem is that chlorine attacks are difficult to prove conclusively through laboratory testing. Sarin leaves distinctive traces but chlorine is a very common element and it's hard to distinguish between chlorine used in a chemical attack and chlorine that exists naturally in the environment.

The balance of other evidence, such as witness statements and the remains of munitions, may strongly indicate a chlorine attack while still leaving room for sceptics to dispute it.

If we look at what is alleged to have happened on 19 May, another problem becomes apparent. Chlorine-filled munitions are said to have injured four rebel fighters. While that is a potential breach of the Chemical Weapons Convention and ought to be investigated, it's difficult to argue that airstrikes would be a proportionate response.

More generally, airstrikes are not a sensible response to chemical attacks. They pre-empt investigations by the OPCW and to some extent undermine them. Breaches of the Convention should be treated in much the same way as war crimes: by meticulously gathering evidence with the aim, ultimately, of prosecuting those responsible.

It's a slow, painstaking process but, over time, it offers the best hope for maintaining the taboo on chemical weapons.

• You can view the research for this story via Write in Stone.