Claims that Russia may have resorted to using chemical weapons during an attack against Ukrainian positions raises important questions about how the West might respond to any further escalation in the conflict.
To date, Nato leaders have been determined not to enter the war for fear of becoming involved in a military confrontation with Russia, a development that could have catastrophic consequences for the rest of Europe.
Nato Secretary General Jens Stoltenberg neatly summed up the continent's dilemma following a recent meeting between members of the security umbrella, when he warned that the alliance was treading a fine line between providing Kyiv with military support it needed, but not to the extent that it risked provoking a direct conflict with Moscow.
It is for this reason that the alliance has been reluctant to provide Ukraine with offensive weaponry, such as tanks and warplanes, preferring instead to concentrate on defensive measures, including anti-tank and anti-aircraft missiles. The Neptune anti-ship missile, believed to have struck Russia's Black Sea flagship Moskva, is a Ukrainian-made weapon based on a Soviet-era anti-ship missile.
But while western leaders are taking extreme caution to prevent the conflict from spreading beyond Ukraine's borders, the issue of chemical weapons – and whether Russian forces might be tempted to employ them as they intensify their efforts to capture the Donbas region of eastern Ukraine – presents a completely different set of challenges.
A decade ago, during the Syrian civil war, the possibility that the Assad regime might use chemical weapons against rebel forces was deemed by the Obama administration and its western allies, such as the UK, to be a "red line" that, if crossed, would prompt a military response.
But when forces loyal to Syrian President Bashar Al Assad were accused of using chemical weapons in an attack on the Damascus suburb of Ghouta in August 2013, the West's failure to respond by launching military strikes seriously damaged its international credibility.
The Ukraine conflict is different in nature, but the issue of chemical weapons is one that western policymakers cannot simply ignore.
To date, they have declined to be drawn on whether they would consider the use of such weapons against Ukrainian forces to be the proverbial red line that obliges them to act. Speaking after a meeting of Nato leaders in March, US President Joe Biden simply stated that his country and its allies would respond "in kind" if such weapons were used, without providing precise details.
Indeed, the longer the conflict continues, there is a deepening understanding among western leaders that, rather than stating their intentions in public, it is to their advantage to introduce a degree of ambiguity in their statements, thereby keeping Moscow guessing as to how they might react to any given situation.
The West's dilemma was brought into focus this week after unconfirmed reports of a possible attack taking place against Ukrainian forces defending the besieged city of Mariupol.
A video published online by Ukraine's Azov Regiment, which is playing a key role in Mariupol’s defence, claimed that a Russian drone had dropped some form of poisonous material on Azov fighters and civilians in the city. The report claimed victims experienced difficulty breathing, muscular function problems, pain and irritation to throat and lungs, and racing heartbeat.
Three people required medical attention, although no one was badly injured, with the worst-affected casualty said to be an elderly civilian.
Concerns that banned weapons might have been used at Mariupol deepened after Eduard Basurin, the leader of Russian separatist forces in the region, had claimed that the only way to defeat the Azov forces was to "bring in chemical troops".
The West was quick to take the reports at face value, with British Foreign Secretary Liz Truss issuing a warning. "Any use of such weapons would be a callous escalation in this conflict and we will hold [Russian President Vladimir] Putin and his regime to account." In Washington, US Secretary of State Antony Blinken claimed that the US had "credible information" that Russia could use some form of "chemical agents" in its efforts to capture Mariupol.
"We have credible information that Russian forces may use a variety of riot control agents, including tear gas mixed with chemical agents, that would cause stronger symptoms to weaken and incapacitate entrenched Ukrainian fighters and civilians, as part of the aggressive campaign to take Mariupol," Mr Blinken said.
To date, there has been no independent confirmation whether last week's alleged attack contained a chemical agent. There is also a chance the report could have been a propaganda exercise on the part of Ukraine's highly effective information warfare centre.
Nevertheless, the incident will have concentrated the minds of western leaders.
The Russian military, it is alleged, has allowed for the use of banned weaponry before – including supposedly in Syria, where chemical weapons of varying descriptions were used against civilian targets in cities such as Aleppo. Moscow has long denied doing so, but human rights groups have accused Syrian government forces fighting alongside the Russians of launching co-ordinated chemical attacks against opposition-controlled parts of the country's north.
Whether or not such tactics will be used in Ukraine, the West is yet unclear about how its leaders might respond without provoking an even bigger conflict with Moscow.
