Ukraine's President Volodymyr Zelenskyy has described Russia's attack on a chemical plant in the country's east as "madness".
His comments came after a Russian airstrike on Sievierodonetsk hit a tank of toxic nitric acid at a chemical factory, causing a huge leak of fumes, according to local authorities.
"Given the presence of large-scale chemical production in Severodonetsk, the Russian army's strikes there, including blind air bombing, are just crazy," Zelenskyy said on Tuesday (Wednesday AEST), reports the AFP news agency.
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"But on the 97th day of such a war, it is no longer surprising that for the Russian military, for Russian commanders, for Russian soldiers, any madness is absolutely acceptable."
Serhiy Haidai, governor of the Luhansk region. posted a picture of a big cloud hanging over Severodonetsk and urged residents to stay inside and wear gas masks or improvised ones.
Haidai said later on Tuesday that "most of Sievierodonetsk" was under Russian control, though he added that fierce fighting continued and the city wasn't surrounded.
Sievierodonetsk is important to Russian efforts to capture the Donbas before more Western arms arrive to bolster Ukraine's defence. Moscow-backed separatists have been fighting Ukrainian troops in the region for eight years and held swaths of territory even before the invasion.
The city, which is 145km south of the Russian border, is in an area that is the last pocket under Ukrainian government control in the Luhansk region. The Donbas is made up of the Luhansk and Donetsk regions.
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Meanwhile, Ukrainian officials have given an update on investigations into suspected Russian war criminals.
Ukraine's prosecutor general, Iryna Venediktova, said that her office has already opened some 15,000 criminal investigations related to the war and identified more than 500 suspects, including Russian ministers, military commanders and propagandists. She said her office was ready to proceed against some 80 of them.
Estonia, Latvia and Slovakia on Tuesday joined an international investigation team probing war crimes in Ukraine.
Last week, in the first case of its kind linked to the war, a Ukrainian court sentenced a captured Russian soldier to the maximum penalty of life in prison for killing a civilian.
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On Tuesday, a court in Ukraine convicted two Russian soldiers of war crimes for the shelling of civilian buildings and sentenced both to eleven-and-a-half years in prison.
Russia staunchly denies its troops are responsible for atrocities. The defence ministry said earlier this month that "not a single civilian has faced any violent action by the Russian military."
– Reported with Associated Press
