Ukraine President Volodymyr Zelensky on Thursday accused Russia of using phosphorus bombs in the latest series of attacks on besieged Ukrainian cities.

“Europe is going through a war, every day of which is full of war crimes of Russian troops. This morning, I received information that Russian troops had used phosphorus bombs against civilians in Ukraine,” the 44-year-old told NATO leaders on Thursday, as per a translation by NBC News.

While the Ukrainian President’s claims are yet to be independently verified, Irpin Mayor Oleksandr Markushin had also accused Russian forces of using phosphorus bombs in the outskirts of Kyiv on March 23.

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Moscow already stands accused of deploying thermobaric missile launchers in Ukraine, while the use of banned cluster munitions has also been independently verified. If the allegations about the use of white phosphorus turn out to be true, it would mark yet another disturbing turn in Russia’s assault on Ukraine.

What are phosphorus bombs?

Phosphorus bombs make use of the white form of phosphorus, which is a highly toxic and incendiary material that ignites on contact with oxygen.

According to international NGO Human Rights Watch (HRW), white phosphorus is not only “notorious for the severity of the injuries it causes,” but can also “burn people to the bone, smolder inside the body, and reignite when bandages are removed.”

The HRW also states that beyond its incendiary uses, even the inhalation of white phosphorus can cause sudden death in certain cases.

In modern warfare, white phosphorus is typically used to produce large amounts of smoke for smoke screens to conceal troop movement.

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However, there have been several documented instances when white phosphorus has been used on people. Prior to the early 2000s, the use of white phosphorus munitions was fairly common, and it was used in the Vietnam War, the two Chechen Wars, and the Falklands War. The US military also used white phosphorus on Iraqis in the Battle of Fallujah in 2004, sparking international condemnation. White phosphorus munitions have also been used in the Israel-Lebanon conflict, in Afghanistan in 2009, in the Syrian civil war, and in the 2020 Nagorno-Karabakh war.

Although military applications of white phosphorus that do not make use of its toxic properties are not prohibited by international law, there have been many calls to strengthen international regulations governing the use of destructive incendiary weapons on people.