Showers of bright, burning light have been falling on Ukraine.
While they can look like fireworks, experts warn these attacks could involve dangerous incendiary weapons.
Such munitions can burn at around 2,000 degrees Celsius and cause excruciatingly painful burns to the skin and permanent scarring.
Vision of fiery rain geolocated at the town of Vuhledar in Ukraine's east has been circulating on Telegram and Twitter in recent days, but experts remain divided on the question of whether it shows incendiary weapons.
But it would not be the first time such ammunition has been used in Ukraine — Human Rights Watch has documented at least 40 attacks where incendiary weapons were deployed over the past year of the conflict.
The group and other experts pointed out these kinds of weapons have been deployed in Ukraine as far back as 2014, and had also been fired in Afghanistan, Syria, Iraq, Gaza, Nagorno-Karabakh and Yemen.
What are incendiary weapons?
Incendiary weapons are designed to set fire that burns for longer than usual due to a particular chemical reaction, according to Human Rights Watch senior researcher Bonnie Docherty.
"Incendiary weapons are weapons that set fire or burn people," she said.
"They're different from chemical weapons, in they that they operate through heat and flame, not through their toxicity."
And they can cause significant damage and deep burns to the skin – sometimes to the bone – with the human rights group describing them as "among the cruellest weapons used in contemporary armed conflict".
Lukas Andriukaitis, a non-resident fellow at the Atlantic Council's Digital Forensic Research Lab, said these munitions "have chemicals inside that upon detonation burn for prolonged periods of time and are very hard to put out".
Tracking social media footage
Human Rights Watch has tracked more than 30 social media videos which they determined showed active incendiary weapon attacks across Ukraine, including in Dnipropetrovsk, Donetsk, Kharkiv, Kyiv, Luhansk, and Mykolaiv oblasts.
Dr Marina Miron, a researcher at the defence studies department at King's College London, has analysed videos showing explosions in Ukraine, and said there was a "high probability" that some social media footage showed incendiary munitions.
In such videos, she observed sparks hitting the ground and producing fire.
Mr Andriukaitis said while it's impossible to be 100 per cent sure judging by a video alone, some footage he has seen in Ukraine was typical of incendiary munitions he had reported on in Syria, Ukraine and Nagorno-Karabakh.
Dr Miron and Ms Docherty pointed out that both Ukraine and Russia had access to these weapons.
Both Dr Miron and Mr Andriukaitis suspected some attacks could have involved thermite, which burns at 2,000 degrees Celsius, although they cautioned that it was difficult to know for sure.
Dr Miron explained that thermite was a mixture of iron oxide and a flammable metal, such as aluminium, magnesium or titanium.
Human Rights Watch said it had mostly detected magnesium alloy in incendiary weapons during the conflict.
Mr Andriukaitis added that in terms of open-source investigation, satellite imagery was a useful tool in proving incendiary weapons had been deployed.
"These weapons usually leave large burn areas clearly visible from space," he said.
What damage do incendiary weapons cause?
"They generally cause both immediate suffering in terms of these excruciating burns, respiratory damage … but also lifelong suffering in terms of scarring [and] psychological harm," Ms Docherty said.
A Human Rights Watch report from 2020 shared the story of Muhammed Assi, who was left badly scarred as a result of an incendiary attack on his school playground in Syria in 2013.
"The hard part is my little nephew is scared to come near me," he said.
A key concern about incendiary munitions was their indiscriminate nature, because of how they were dispersed, Dr Miron said.
"For instance, the warhead [of the likes of 9M22S] consists of 180 incendiary elements which create that 'star shower' effect," she said.
"It is not possible to control where they fall. So it is difficult to ensure that no civilians are harmed.
"Should a spark hit a person, there is a high possibility of severe burns [with possible deadly consequences] and respiratory injuries."
Mr Andriukaitis said such weapons were "not precise and cause high collateral damage to the area" by starting fires.
"The burn marks that they leave on human bodies are horrible and very hard to treat," he said.
"They are often used as a weapon of terror."
It could also have a socio-economic impact in terms of preventing people from returning to work due to stigmatisation or injury.
Ms Docherty said Human Rights Watch had yet to document any civilian harm caused by incendiary weapons in Ukraine, but the group was monitoring this.
What are the loopholes in international law?
Given the impacts on civilians, Dr Miron said incendiary weapons were only allowed to be used against military targets.
"I think it is disconcerting that these weapons are still being used in areas where civilians are located," she said.
Under the Protocol III to the Convention on Conventional Weapons (CCW), air-dropped incendiary munitions are forbidden in populated areas, but ground-fired incendiary weapons are allowed in some circumstances.
Ms Docherty said these weaker regulations for ground-fired weapons was a loophole that needed to be closed.
"If indeed these are incendiaries falling in populated areas, that is a problem," she said.
Another loophole, she said, was that the protocol didn't cover multipurpose munitions – such as white phosphorus – because it was "primarily designed" to cause a smokescreen.
For example, military forces might use white phosphorus on an open battlefield to obscure the movement of tanks and weapons, and they are therefore not classified as incendiary weapons, even though they have the same incendiary effects.
When used as intended they are not illegal, but when fired at civilian targets they are indiscriminate and a war crime.
Human Rights Watch has said that white phosphorus is "notorious" for the severe injuries – it "can burn people to the bone, smoulder inside the body, and reignite when bandages are removed".
Ms Docherty said Human Rights Watch hasn't recorded white phosphorus in Ukraine yet, but it has been used by US-led forces against the Islamic State in Iraq and Syria in 2017 and also by Saudi Arabia-led forces in Yemen in 2016, for example.
In a joint statement last year, more than three dozen countries, including Australia, expressed concern about alleged violations of the Convention on Conventional Weapons.
"We are gravely concerned about increasing reports of Russia's failure to comply with its obligations under the CCW and its Protocols during its manifestly unlawful war of aggression against Ukraine," the statement said.
