The Foreign Secretary has vowed to hold Vladimir Putin “and his regime to account” if it is proven Russian forces used chemical agents in an attack on Mariupol.
The city’s Azov regiment - a unit with far-right links which is defending the city - reported soldiers were left dizzy and unable to breathe after a “poisonous substance of unknown origin” was dropped on them from a Russian drone.
The Kyiv Independent news website reported that Azov leader Andriy Biletsky said that three people have clear signs of chemical poisoning.
It came hours after Mariupol’s mayor said more than 10,000 civilians have died in the Russian siege of his city and the death toll could surpass 20,000.
Western leaders warned even before Russian troops moved into Ukraine in February that Russia could resort to unconventional weapons there, but so far there have been no confirmed reports of their use.
Here is everything you need to know.
What are chemical weapons?
Chemical weapons are different to “regular” weapons, like bombs, in that they do not explode, instead using poisonous chemical compounds to injure or kill people.
Since the turn of the 20th century, several international laws have said that using chemical weapons is illegal, but such weapons have still been used in wars since then - for example, Germany’s use of poison gas during World War I.
About 70 different chemicals are thought to have been used or stockpiled for use as chemical warfare agents during the 20th century; the Chemical Weapons Convention - created in 1997 - says that all of these chemicals should be destroyed.
There are a number of different types of chemical weapons, as well as a wide spectrum on the damage they can cause, depending on their specific use.
UK armed forces minister James Heappey told Sky News: “It’s important to recognise that there are all sorts of ways in which these things could be used.
“From the use of tear gas which is effectively a riot-control measure, all the way through to utterly devastating lethal chemical weapons systems, so I don’t think it’s helpful to be too binary about the situation because these are highly nuanced.”
Which chemical weapons could have been used in Ukraine?
Britain is increasingly worried that Russia could use white phosphorus munitions in the bombardment of the city.
White phosphorus can be used for tactical reasons, for instance for illumination at night or to create a smokescreen; when it is deployed as a weapon it causes horrific burns.
UK armed forces minister James Heappey has said the use of chemical weapons is “beyond the pale”, and if the reports were accurate there would be a response from the UK and its allies.
But it’s not just direct chemical attacks that are of concern.
Late on Monday (11 April), Ukraine’s parliament said Russian forces had fired on nitric acid tanks in Donetsk, with residents of the eastern city urged to prepare “protective face masks soaked in soda solution”.
These indirect attacks - using conventional weapons to trigger the release of toxic chemicals already housed within Ukraine - could be seen as Russia attempting to exploit loophole in the rhetoric.
If this were the case, Russia may not have unleashed chemical weapons attacks on the country directly, but by attacking chemical repositories and releasing the substances within, could still be achieving similar effects.
Of course, any such move would still be frowned upon heavily by the West, and would most likely be treated similarly to a more direct chemical attack in its response.
The Geneva Convention provides a general protection of civilian objects - attacks must be on military objectives and justified by military necessity; the harm caused to civilians or property must be proportional and not excessive.
A deliberate attack on gas and chemical supplies, leading to widespread civilian suffering, would likely grab the attention of Iryna Venediktova, Ukraine’s prosecutor general investigating Russia’s alleged war crimes.
The UK will work with allies to “investigate war crimes and ensure justice is done”, a Foreign Office spokesman said.
Why would Russia use chemical weapons?
Western officials think Russia wants to bring about the fall of Mariupol to both free up troops for the fight in the Donbas, but also to create a route north for the Kremlin’s forces as they look to form a pincer movement on Ukrainian defenders in the east.
A Russia-allied separatist official, Eduard Basurin, appeared to urge their use Monday (11 April), telling Russian state TV that Russian-backed forces should seize a giant metals plant in Mariupol from Ukrainian forces by first blocking all the exits out of the factory.
“And then we’ll use chemical troops to smoke them out of there,” he said.
Officials have said Putin will double or even possibly triple the number of Russian troops in the Donbas as the Russian president resorts to a “diminished” invasion strategy.
The amassing of troops, however, will not necessarily give Moscow an advantage over Ukraine, with Kyiv’s forces having had success in pushing back insurgents in the east of the country, they said.
The Russian leader has been forced to “diminish considerably” the plan in Ukraine, one official said, amid suggestions Putin wants to take the Donbas region before 9 May - when Russia traditionally marks the Soviet Union’s Second World War victory against Nazi Germany with military parades in Moscow - in an attempt to claim victory for his so-called “special operation”.
How would the West respond?
The West will have “all options on the table” if Putin’s forces are found to have used chemical weapons in an assault on the Ukrainian port of Mariupol, UK armed forces minister James Heappey said.
He added “there are some things that are beyond the pale and the use of chemical weapons will get a response, and all options are on the table for what that response could be”.
Foreign Secretary Liz Truss said the UK was working urgently with partners to verify details of the attack, adding: “Any use of such weapons would be a callous escalation in this conflict and we will hold Putin and his regime to account.”
Shadow foreign secretary David Lammy said: “If chemical weapons have been used as part of Vladimir Putin’s already barbaric and illegal invasion of Ukraine, this would represent an appalling escalation and amount to a war crime.