The US, UK and its allies have been voicing their concern about Russia using chemical weapons in its war against Ukraine. Hereâ€™s a guide to what they are and whether their use would represent an escalation of tensions that widens the conflict.
What are chemical weapons?
The Organisation for the Prohibition of Chemical Weapons, the global watchdog that oversees implementation of the Chemical Weapons Convention (CWC), says a chemical weapon is a chemical used to cause intentional death or harm through its toxic properties. The 1997 convention bans developing, using or stockpiling chemical weapons, and is signed by 193 countries, including Russia.
Chemical weapons come in gas, liquid form or solid forms, and may be included in bombs and other such munitions. They are categorised by the effect they have on the human body. Nerve agents, for instance, work by blocking messages from the nervous system to the muscles. There are also blistering, choking and blood agents.
What are the types of chemical weapons?
Types of nerve agents include sarin, soman and tabun. Sarin is a man-made nerve gas that was originally developed as a pesticide in Germany in 1938. It is chemically similar to a class of pesticides known as organophosphates.
Sarin, also known as GB, is part of a class of chemical weapons called G-series nerve agents that were developed during the Second World War and were named for the German scientists who synthesized them. At room temperature, G-series nerve agents are volatile liquids, with sarin being the most volatile.
Novichok, which means â€œnewcomerâ€ in Russian, applies to a group of military-grade nerve agents that the Soviet Union developed in the 1980s. It is made with organophosphates, and is believed to be much more toxic than other known nerve agents.
By contrast, biological weapons deliberately spread viruses, bacteria, fungi or toxins in order to cause harm
When have they been used?
The modern use of chemical weapons began during the First World War, when both sides used poisonous gas to inflict suffering and to cause significant battlefield casualties. This include sulphur mustard, commonly referred to as â€œmustard gasâ€.
The most notable use of chemical weapons in history was by the Nazis in concentrations camps during the Holocaust, during which they were administered in gas chambers.
Sarin and other nerve agents may have been used in chemical warfare during the Iran-Iraq war in the 1980s.
What happened in Syria?
The Syrian government has been accused multiple times of carrying out chemical weapons attacks on civilians since the civil war began in 2011.
In 2018, the BBC reported that at least 106 chemical attacks had taken place in Syria since September 2013, when president Bashar al-Assad signed the CWC and agreed to destroy the countryâ€™s chemical weapons stockpile.
In 2013, horrific pictures of victims a chemical attack in Ghouta outside Syriaâ€™s capital, Damascus, shocked the world. Syriaâ€™s opposition claimed the attack killed up to 1,300 people, many of them children. Western powers said the attack could only have been carried out by the government, but Assad blamed the opposition. His regime is backed by Russia, who has also denied the claims.
The use of chemical weapons by the Syrian regime was termed a â€œred lineâ€ by then US president Barack Obama in 2012. But he was unable to gather a coalition of allies to punish Assad by using military force.
What happened in the UK?
Russia was accused of conducting a chemical attack in Salisbury in 2018, injuring five people including target Sergei Skripal. A woman who was killed by the Novichok nerve agent following the attempted murder of former Russian double agent Skripal.
Dawn Sturgess died from exposure to Novichok in July 2018 after her partner Charlie Rowley found a counterfeit perfume bottle which police believe had been used by Russian intelligence operatives to smuggle the poison into the country.
Skripal, who sold Russian secrets to Britain, and his daughter Yulia were found slumped unconscious on a public bench in the English city. They and a police officer who went to Skripalâ€™s house were left critically ill in hospital from exposure to the nerve agent.
What is Russia claiming?
Russian has claimed â€“ without providing evidence â€“ that Ukraine ran biological weapons laboratories with US support.
Under a 2005 agreement, the Pentagon has assisted several Ukrainian public health laboratories with improving the security of dangerous pathogens and technology used to research. Those efforts have been supported by other countries and the World Health Organisation.
The US envoy to the United Nations, Linda Thomas-Greenfield, said Washington was â€œdeeply concernedâ€ that Russia is deploying a â€œfalse flag effortâ€ aimed at laying the groundwork for its own use of biological or chemical weapons in Ukraine.
What has the West said?
Some have speculated whether Russia using chemical weapons would represent another â€œred lineâ€. Direct military engagement between the US and Russia â€“ the worldâ€™s two biggest nuclear powers â€“ would represent a significant escalation of the war.
Britain has warned Vladimir Putin of a â€œdramatic increased responseâ€ from the West if he uses chemical weapons in Ukraine. Prime minister Boris Johnson has expressed fears Russia is spreading a â€œfake storyâ€ that the US or the Ukrainians have chemical weapons in Ukraine as a pretext for an atrocity.
â€œThe stuff which you are hearing about chemical weapons is straight out of the Russian playbook,â€ he told Sky Newsâ€™ Beth Rigby Interviews on Thursday.
â€œThey start saying that there are chemical weapons that are being stored by their opponents or by the Americans so that when they themselves deploy chemical weapons, as I fear they may, they have a fake story ready to go.â€
On Friday, US president Joe Biden said that Russia will pay a â€œsevere priceâ€ if the country uses chemical weapons in Ukraine. â€œIâ€™m not going to speak about the intelligence, but Russia will pay a severe price if they use chemicals,â€ he said.