President Volodymyr Zelensky of Ukraine said he couldn’t say for sure if chemical weapons were used in Mariupol.

“We are deeply concerned about yesterday’s reports of the use of poisonous munitions against the city’s defenders in Mariupol. “It is still too early to say what kind of substance it was,” Zelensky said via video on Tuesday. “It is clear that a full investigation and analysis are impossible in the blocked city.”

Zelensky warned other leaders to be wary of the use of chemical weapons.

“However, given the repeated threats by Russian propagandists to use chemical weapons against the defenders of Mariupol, the world must respond now,” he warned. “Be proactive. Because responding to the use of weapons will not change anything. And it will be a humiliation for democracy.”