President Volodymyr Zelenskyy of Ukraine said on Wednesday that the recent swell of shelling in Mariupol has left up to 100,000 civilians trapped in the city with no clear route to safety amid “constant bombing".
After previously confirming its use of highly destructive and illegal weapons in Ukraine such as vacuum bombs, fears over the possibility of more severe weapon attacks are growing.
A spokesperson for President Putin said on Tuesday that the country would only use nuclear weapons should its very existence come under threat, as US President Joe Biden warned of the likelihood for chemical weapon attack as part of Russian false flag operations.
The extent of destruction seen in Ukraine’s major cities has drawn comparisons with obliterative attacks in Syria, prompting further concern over the potential use of chemical weapons.
Here’s what a chemical weapon is, how they differ to biological weapons and why experts believe they may be used by Russia in Ukraine.
What are chemical weapons?
Chemical weapons are those which deploy toxic chemicals via missiles, aircraft or artillery shelling to cause injury, impairment or death.
They typically fall into the categories of blister agents, blood choking agents, choking agents and nerve agents, with categories determined by the areas of the body targeted and affected by different types of chemical weapons.
Nerve agents are perhaps the most commonly used chemical weapons in modern warfare, targeting and causing severe damage to the central nervous system when breathed in as gas or absorbed through the skin in liquid form.
Sarin gas is a well known example of a nerve agent, first developed by Germany in 1938 as a pesticide but deployed in fatal attacks such as the Tokyo subway sarin attack of 1995 and more recently in Syria in 2013 and 2017.
Blister agents like mustard gas, used widely in World War I, cause extreme blistering on the lungs and body when inhaled or absorbed, while choking agents such as chlorine gas can cause respiratory failure by attacking the respiratory system.
Tear gas is also considered a chemical weapon when used in warfare, this and fellow riot control agents such as pepper spray can be legally owned and used by certain countries.
Despite being banned by international law, and restricted by the creation of the Chemical Weapons Convention in 1993, a number of states refused to sign the Convention and maintain supplies of chemical weapons – alongside signatories such as Russia.
These were North Korea, South Sudan and Egypt.
How are they different to biological weapons?
While chemical weapons are deployed by aircraft, shelling or rockets to have an instant impact and deteriorating effect on important parts of the body, biological weapons take a much longer time to have an effect on the body.
Biological weapons are toxins and toxic microorganisms like bacteria and viruses which can be harnessed as weapons when released at scale, resulting in severe illness, incapacitation or death.
Examples include toxins like ricin, bacteria such as anthrax, ebola virus and diseases such as small pox.
Does Russia have chemical weapons?
Despite being a signatory of the Chemical Weapon Convention, ratifying in 1997, Russia is believed to maintain a supply of chemical weapons.
In 1997 it declared holdings of 40,000 tons of chemical weapons, with 32,200 tons of these accounting for nerve gases including sarin.
In 2017 Russia said that its chemical weapons had been destroyed, but this came after a rival and critic of Vladimir Putin, Alexei Navalny, was found by the Organisation for the Prohibition of Chemical Weapons (OPCW) to have been poisoned by a Soviet-era nerve agent known as Novichok.
The nerve agent had been developed at a state-owned chemical research facility in Russia between the 1970s and 80s.
It was also used in the 2018 Salisbury poisonings of former Russian military officer and British double agent Sergei Skripal and his daughter, Yulia Skripal.
Has Russia used chemical weapons in Ukraine?
Amid the continued shelling and bombardment of Ukraine and confirmed use of a thermobaric weapon, chemical weapons have not yet been used in Ukraine – but concerns over President Putin’s potential use of them are increasing.
This comes after Russian leaders claimed Ukraine is developing and preparing to use chemical weapons of its own – in a statement that has been widely declared a false flag operation and one potentially designed to create a pretext for its own use of chemical weapons.
On Tuesday (March 22), President Joe Biden said that Russia’s struggle to seize major cities in Ukraine and declare an easy victory could see President Putin turn to chemical weapon attacks.
"Putin's back is against the wall,” said President Biden.
"He wasn't anticipating the extent or the strength of our unity.
"And the more his back is against the wall, the greater the severity of the tactics he may employ.
The US President added: “Whenever he starts talking about something he thinks Nato, Ukraine, or the United States is about to do, it means he's getting ready to do it."
"Now he's talking about new false flags he's setting up, including he's asserting that, we, in America, have biological as well as chemical weapons in Europe - simply not true.
"They're also suggesting that Ukraine has biological and chemical weapons in Ukraine.
"That's a clear sign he is considering using both of those.
"He's already used chemical weapons in the past, and we should be careful of what's about to come.
"He knows there'll be severe consequences because of the united Nato front, but the point is: It's real."
Likewise, UK Prime Minister Boris Johnson said that such a strategy would be “straight out of Russia’s playbook” in an interview with Sky News on Thursday March 10.
"The stuff that you're hearing about chemical weapons is straight out of their playbook," Mr Johnson told Sky’s Beth Rigby.
"They start saying that there are chemical weapons that have been stored by their opponents or by the Americans.
"And so when they themselves deploy chemical weapons, as I fear they may, they have a sort of maskirovka - a fake story - ready to go.
"You've seen it in Syria. You saw it even in the UK. That's what they're already doing. It is a cynical, barbaric government."
Putin previously claimed the Khan Shaykhun sarin gas attack, which killed almost 100 people in a town in north-west Syria on April 4 2017, found to have been carried out by President Bashar Hafez al-Assad’s regime, was staged by the West.
Chemical weapons expert Hamish de Bretton-Gordon told BBC News on Friday March 10 that if Russia continues to echo its attacks on Syria, the possibility of chemical weapon use is high.
“Chemical weapons have been used extensively in Syria,” he said.
"The possibility that they are used in Ukraine, I think, is quite high because they are morbidly brilliant weapons and if you have no morals or scruples, you would use them all the time.
"And the Russians are showing that they don't have much of either at the moment.”