Ukraine has accused invading Russian troops of using phosphorus bombs -- incendiary weapons whose use against civilians is banned under an international convention but allowed for military targets.
Phosphorus weapons, which leave a signature white trail in the sky, were deployed against a village in the Lugansk region and at Irpin outside Kyiv, Ukrainian officials including President Volodymyr Zelenskyy have claimed.
It was not immediately possible to verify the allegation.
"Russia has never violated any international convention," Kremlin spokesman Dmitry Peskov retorted.
Phosphorus, a substance that catches fire on contact with the air, "is not classified as a chemical weapon, it's available to many of the world's armies," Olivier Lepick, a researcher with France's Foundation for Strategic Research (FRS), told broadcaster LCI on Thursday.
It is used to create smokescreens to hide troop movements, illuminate the battlefield or destroy buildings by fire -- but can "cause absolutely horrific damage, extremely large burns" if it hits people, he added.
As an incendiary, it is not covered by the Convention on Chemical Weapons which entered into force in 1997.
However, it does fall under the 1983 Convention on Certain Conventional Weapons, which restricts fire weapons without forbidding their use altogether.
Both Russia and Ukraine are considered signatories, as parts of the former Soviet Union.
The use of incendiaries against civilians and non-military targets is "prohibited in all circumstances" as well as their deployment against military targets near civilians.
But phosphorus is not covered by the convention when used for smoke screening or battlefield illumination.
World War I saw the first widespread use of incendiary weapons, just as the air force began to play a significant military role.
In May 1915, a German Zeppelin airship dropped firebombs on London.
White phosphorus shells were widely used in World War II, especially by American troops fighting German armoured forces in the European theatre.
Invented in 1942, napalm -- a form of thickened petrol -- was widely used by US forces against Vietcong guerilla fighters in the Vietnam war, with widespread civilian casualties.
Then-colonial power France had earlier used the fire weapons in its own battles in Indochina.
After the turn of the millennium, US forces were accused of using phosphorus bombs in their 2004 assault on the Iraqi city of Fallujah.
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Then-military chief Peter Pace said that targeting insurgents with weapons was a "legitimate tool of the military".
"It is not a chemical weapon. It is incendiary. And it is well within the law of war to use those weapons" for smokescreens and illumination, he insisted.
The head of the UN's aid agency for Palestinians accused Israel of using white phosphorus in Gaza in 2009.
And Syrian observers charged that Russia used incendiaries in Syria against Eastern Ghouta, a rebel stronghold near Damascus -- which Moscow denied.
The Kremlin has itself accused Ukraine's army of phosphorus use in 2014, against Russia-backed separatists in the eastern Donbas region.
And Armenia and Azerbaijan each accused the other of bombing civilian areas or using banned phosphorus weapons in their war over Nagorny Karabakh in 2020.
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