According to the Ukrainian claims, Moscow has used Phosphorus bombs in several areas.
Phosphorus bombs contain a mixture of white phosphorus and are not banned under the chemical weapons under the international convention; rubber can either be used as incendiary weapons or create smoke screens. They can be used on battlefields, but the use of these bombs is not to be used in civilian areas.
The mixture of white phosphorus and the rubber ignites in contact with oxygen in the air. A flame of up to 1,300 degrees Celsius (2,372 degrees Fahrenheit) is generated and is then accompanied by dense white smoke.
On March 24, Ukrainian President Voldomyr Zelensky said in an address to NATO that the "Russian Phosphorus bombs had been used that morning- and had killed many adults and children" he didn't provide any further details. Even Ukrainian official Anton Gerashchenko has said that the bombs were used in Mariupol in May.
Whether Russia has used phosphorus bombs cannot be denied or disproved at this stage. However, there are various pictures and videos which are floating around that suggest that they might have used phosphorus bombs.