The United States, United Kingdom, and Australia have said they are working to verify unconfirmed reports on social media that Russian forces used a “poisonous substance” in the long-contested southern city of Mariupol. The Azov Regiment, an ultra-nationalist unit of the National Guard of Ukraine, said on Telegram that Russia used a “poisonous substance of an unknown origin” in the port city. The regiment has been defending war-torn Mariupol, a strategic port city on the Azov Sea that would provide Russia a land corridor to Crimea if captured, since it took control from pro-Russian separatist forces in 2014. “Russian occupation forces used a poisonous substance of unknown origin against Ukrainian military and civilians in the city of Mariupol, which was dropped from an enemy [unmanned aerial vehicle],” the Azov Regiment posted on April 11. “The victims have respiratory failure, vestibulo-atactic syndrome. The consequences of using an unknown substance are being clarified.” The …