President Volodymyr Zelenskyy warned that Russia could use chemical weapons in Ukraine and called on the West to impose strong sanctions on Moscow that would deter even talk of the use of such weapons. This comes amidst unconfirmed reports on Monday suggesting that chemical weapons were used in the besieged southern Ukrainian port of Mariupol. "We treat this with the utmost seriousness," Zelensky said in his nightly video address on Monday. Watch this video.