Russia denies possessing chemical weapons and poisoning Russian dissidents in the UK with chemical nerve agents, charges made by the US, UK and Europe.
White House press secretary Jen Psaki had previously suggested Russia could stage a “false flag attack” or spread conspiracy theories to blame Ukraine.
"We should all be on the lookout for Russia to possibly use chemical or biological weapons in Ukraine, or to create a false flag operation using them – it's a clear pattern," Ms Pskai said on Twitter.
But what are chemical weapons, why are they controversial and what advantages might they have on the battlefield?
When were chemical weapons developed?
The Organisation for the Prohibition of Chemical Weapons (OPCW) describes them as "munitions, devices and other equipment specifically designed to weaponise toxic chemicals".
First used at the battle of Ypres in the First World War in 1915, they are banned under the 1993 Chemical Weapons Convention, which has been signed by 193 countries including several that have declared possessing them (the US, Iran) and countries accused of using them in wartime (Syria).
The weapons have been seen by many as a red line in modern warfare because they kill indiscriminately and horrifically.
Victims suffer muscle failure and paralysis, suffocating as their lungs fail, may lose bodily control, vomiting before death or suffer gruesome injuries including severe blistering to the skin, or blindness.
They were used to devastating effect when German forces used canisters filled with mustard gas at a 6-kilometre French and British defensive line at Ypres, Belgium, also overwhelming allied Algerian soldiers.
The scale of the attack is vital in understanding how deadly these weapons of mass destruction can be: at least 1,000 soldiers died from chlorine asphyxiation on the front line.
There are three broad categories of chemical weapons. Nerve agents such as Tabun block an enzyme in the body that controls the muscles, causing cramping that stops the lungs and other organs from functioning. Blood agents, such as cyanide, stop cells from functioning properly, eventually causing death.
Then there are blistering agents, such as the mustard gas commonly used in the First World War. Unlike nerve and blood agents, some of which can kill people in seconds in tiny doses, blistering agents are much less likely to kill but cause lung and skin damage and even blindness.
Are they illegal?
Chemical attacks became common during the First World War and are thought to have killed 90,000 soldiers on all sides but the 1925 Geneva Protocol prohibited their use.
When the Second World War broke out, rival armies refrained from using lethal gas – by this point, the Germans had developed nerve gas, a deadly substance that stops nerve signals and cell function in the body.
While the Geneva Protocol sought to stop their use in wartime, further treaties attempted to prevent development of the weapons during the tension of the Cold War, when the US and former Soviet Union were developing terrifying weapons, including nuclear missiles and biological weapons that kill through disease.
The 1972 Biological and Toxin Weapons Convention sought to stop countries possessing any toxic chemical that could have no peacetime use – but did not allow for international monitoring.
That changed in 1993 when the Organisation for the Prohibition of Chemical Weapons was set up to monitor the destruction of chemical weapons stockpiles among signatories of the CWC.
Have chemical weapons been used in recent years?
It wasn’t until 1984 that a large-scale chemical weapons attack would happen again, when Iraqi forces dropped sulfur mustard and nerve gas agents such as Tabun on Iranian troops around Majnoon near the border with Iran.
The US State Department believed Iraq had used chemical weapons even before then, reporting in 1983 that Baghdad had developed the weapons "with the essential assistance of foreign firms”.
Washington initially condemned the use of the weapons but continued to support Iraq throughout the conflict with Iran, along with European and Russian companies which supplied chemicals and delivery systems.
Iraq would go on to use chemical weapons most infamously at Halabja in the Kurdish region of Iraq in 1988, killing between 3,000 and 5,000 civilians.
After the invasion of Kuwait in 1991, Iraq said it had destroyed its chemical weapons arsenal, despite insistence by the US and UK that some had been retained. That was one of the reasons given for the 2003 US-led invasion, sparking a bitter rift with UN inspectors.
Chemical weapons made headlines again in August 2013, when Bashar Al Assad used them against rebel forces in the Syrian civil war. At Ghouta, hundreds of – and perhaps up to 2,000 – civilians died in a Syrian government chemical attack.
Further use of the weapons by Assad led to the UK, US and France bombing Syrian research centres in April 2018.