Petro Andriushchenko, an adviser to the city’s mayor, reportedly spoke of fires burning up to 2,500 degrees Celsius. He added that “the fire is almost unstoppable”.
Mr Andriushchenko reportedly posted a video of the bombing of the Azovstal steel plant in Mariupol on Telegram.
He said experts should investigate the cause of the burning hell, but the Russians themselves claim to have used incendiary shells.
Phosphorus bombs are a chemical weapon made up of phosphates, which have the ability to ignite on contact with air and burn up to 2,760 degrees according to the US military.
White phosphorus – or “Willie Pete” after its initials – is not banned from use under the Chemical Weapons Convention, but is supposed to be strictly regulated under international law.
Phosphorus bombs have the ability to spread white-hot fire over a wide area, and their chemical properties mean victims can suffer deep burns and organ failure from inhaling fumes.