Russian forces may employ the use of phosphorous bombs in eastern Ukraine as Vladimir Putin’s army prepares to escalate military actions in the region, the British Ministry of Defense warned on Monday.

Phosphorus is highly toxic and can cause severe burns on human flesh. Under international law, phosphorus munitions are prohibited from use near civilians, but Russia’s reliance on unguided bombs in Ukraine may put the civilian population in cities such as Mariupol at risk of death or injury from the incendiary weapon, the British Ministry of Defense said.