The Azoz regiment, a unit of Ukraine’s National Guard, said the chemicals were of “unknown origin” and fell through an unmanned aerial vehicle (UAV) on civilians in the southeastern city of Mariupol. “About an hour ago, the Russian occupation forces used a poisonous substance of unknown origin against Ukrainian soldiers and civilians in the city of Mariupol, which was shot down by an enemy UAV,” the Azoz Regiment told the Telegram on Monday. “The victims have respiratory failure, vestibular syndrome. The consequences of using an unknown substance are being clarified.” NATO has warned that the use of chemical weapons could provoke further action from the West. Late last month, NATO Secretary-General Jens Stoltenberg said the use of chemical weapons was a “gross violation of international law with far-reaching consequences”. President Joe Biden has previously said that such an act could trigger a backlash from the United States, although he has stopped saying what that could be. A Ukrainian military unit said Monday that Russian forces had used chemical weapons against civilians. Above, a Ukrainian soldier emerges from a destroyed building after being bombed by Russian forces in Kyiv on March 12, 2022. ARIS MESSINIS / AFP / Getty Images Newsweek could not independently verify Monday’s allegations. However, several Ukrainian officials also posted about the allegations on social media. Solomiia Bobrovska, a member of the Ukrainian parliament and secretary of the Foreign Affairs Committee, called on Biden and NATO to respond to the reports. “The #Azov Battalion reports that Russia has used an unknown chemical substance against #Mariupoland civilian defenders. Anton Gerashchenko, an adviser to Ukraine’s interior minister, wrote on Twitter about the alleged use of chemical weapons. “ATTENTION! Chemical weapons are being used against Ukrainian defenders in #Mariupol! Russia is crossing all the borders of humanity and is openly declaring it, while Ukraine is still demanding heavy weapons,” he wrote. ATTENTION Chemical weapons are used against Ukrainian defenders in #Mariupol! Russia openly crosses all borders of humanity and declares it openly, while Ukraine continues to demand a heavy weapon #UkraineUnderAttack – Anton Gerashchenko (@Gerashchenko_en) April 11, 2022 Ivanna Klympush, a politician leading the nation’s efforts to integrate Ukraine into the European Union, also condemned the alleged actions and called for further sanctions against Russia. “This is the red line beyond which 🌎 must destroy the despotism economy. We demand a full embargo on all fuels from #RU & 2UA heavy weapons now!” He wrote. 1/2 #RU 1.5 hours ago an unknown substance was used in #Mariupol. Victims experience respiratory failure, vestibular-tactical syndrome. Most likely chemical weapons! This is the red line beyond which 🌎 must destroy the economy of despotism. We demand a full embargo on all fuels from #RU & 2UA heavy weapons now! – Ivanna Klympush (@IKlympush) April 11, 2022 The United Kingdom Ministry of Defense warned on Monday that Russia could soon use phosphorous bombs again as it seeks to increase military action in eastern Ukraine. “Previous use of phosphorus munitions by Russian forces in the Donetsk region increases the likelihood of their future employment in Mariupol as fighting for the city intensifies,” the ministry said in a recent defense update posted on Twitter. Monday’s claim by the battalion is not the first time Russia has been accused of developing dangerous substances in the war that began in late February. Ukrainian officials claimed last month that in the eastern city of Kramatorsk, Russia had developed white phosphorus munitions, a substance strong enough to burn human flesh. White phosphorus is not considered a chemical weapon under the UN Chemical Weapons Convention, but its use is strictly regulated by international law. It is illegal to use it on citizens. Newsweek contacted the Russian Foreign Ministry for comment.