Volodymyr Zelensky, the president of Ukraine, said he couldn’t say for sure whether chemical weapons were used in the besieged city of Mariupol.

“We take with great attention yesterday’s reports on the use of munitions with a poisonous substance in Mariupol against the defenders of the city. It is not yet possible to draw 100% conclusions about what kind of substance it was,” Zelensky said in a video address on Tuesday. “It is clear that it is impossible to conduct a full investigation and full analysis in the blocked city.”

Other leaders, Zelensky cautioned, should be aware of the possibility of chemical weapons being used.

“However, given the repeated threats of Russian propagandists to use chemical weapons against the defenders of Mariupol and its repeated use by the Russian army, such as phosphorus munitions in Ukraine, the world must respond now,” he cautioned. “React preventively. Because after the use of weapons of mass destruction, any response will not change anything. And it will only look like a humiliation for the democratic world.”

Reports that chemical weapons were used in Mariupol have not yet been confirmed by the United States, State Department spokesperson Ned Price said.

Secretary of State Antony Blinken stated on Tuesday that the US could not confirm the use of chemical weapons in Ukraine, but they “had credible information that Russian forces may use a variety of riot control agents.”