What are chemical weapons, why are they banned and will Russia use them?

US and Nato have accused Russia of planning to use them in Ukraine

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  Soldiers from the 82nd Airborne Division patrol in protective masks during Operation Desert Shield in Iraq. Getty Images

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  Senior Airman Antonio Gant dons mission-oriented protective posture (MOPP) gear during training at Yokota Air Base, Japan. Photo: US Air Force

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  Soldiers of the US Army 23rd chemical battalion wear gas masks during a competition to test individual soldier skills at Camp Stanley in Uijeongbu, South Korea. Reuters

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  A Russian chemical and biological defence unit undergoes rescue training after a simulated attack at a military base outside Moscow, in 2002. Reuters

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  Russian soldiers at Serpukhov's military missile research institute in 2010. AFP

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  Polish special forces in a joint training exercise of simulated chemical attacks in Warsaw underground, Poland, in 2015. Reuters

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  Jordanian and US soldiers take part in an exercise to conduct a chemical attack drill in Zarqa, east of Amman, Jordan, in 2018. Reuters

On Tuesday, US President Joe Biden repeated the claim that Russia might resort to using chemical weapons to overcome strong Ukrainian resistance to its invasion.

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?

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  During the Second Battle of Ypres, the Germans released more than 160 tonnes of chlorine gas at the front, in the first deadly gas attack of the war. Getty Images

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  German Red Cross members treat victims of a gas attack during the First World War. AP Photo

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  German soldiers after a gas attack during the First World War. Getty Images

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  In the Second World War, they had gas masks to guard against noxious fumes.

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  Museum staff look over an officer's glove shrunk by poison gas at the First World War Galleries exhibit at the Imperial War Museum in London. EPA

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  Kaylene Biggs, great-granddaughter Australian soldier Andrew Bayne, places a wooden cross with a poppy and a message on the grave at Westhof Farm Commonwealth Cemetery in Nieuwkerke, Belgium. AP Photo

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?

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  Syrian children and adults receive treatment for a suspected chemical attack at a makeshift clinic on the outskirts of Damascus on February 25, 2018. AFP

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  Members of a family from Douma, Syria, who fled after enduring weeks of bombing, near-starvation from a crippling siege and a suspected chemical attack, stand outside a tent in a northern Syrian displaced-persons camp on May 29, 2018. AP

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  UN vehicles carrying the team of the Organization for the Prohibition of Chemical Weapons arrive at a hotel in Damascus on April 14, 2018, hours after the U.S., France and Britian launched an attack on Syrian facilities for suspected chemical attack against civilians. AP

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  Syrian authorities distributed bread, vegetables and pasta to residents of Douma, the site of a chemical weapons attack suspected to have been carried out by the Syrian government, on April 16, 2018. AP Photo

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  A Syrian boy holds an oxygen mask over the face of an infant at a make-shift hospital following a reported gas attack on the town of Douma, Syria, on January 22, 2018. AFP

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  A child receiving oxygen through a respirator following an alleged poison gas attack in Douma on April 8, 2018. Syrian Civil Defense White Helmets via AP

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  The headquarters of the Organisation for the Prohibition of Chemical Weapons, The Hague, Netherlands. AP

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  A civil defence member breathes through an oxygen mask after the sarin gas attack on Khan Sheikhoun on April 4, 2017. Reuters

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”.

(FILES)--In this photograph made available by the official Iraqi presidential photographer 26 April 2002, Iraqi President Saddam Hussein is seen at the Iraq-Iran border during Iraq-Iran war 1980-1988. Saddam will celebrate his 65th birthday on 28 April. He was born in 1937 in Takrit, in the province of Salahedin, 170 kms north of Baghdad. AFP PHOTO/HO (Photo by PRESIDENTIAL PALACE / AFP)

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.