Russian air strikes hit a tank with nitric acid at a chemical plant in Severodonetsk, a city in eastern Ukraine, according to Luhansk's regional governor, Serhiy Haidai.

Haidai is urging residents to stay inside and to wear protective face masks.

Haidai also said Russian forces have made significant gains and are in control of "most" of Severodonetsk.

He said about 60% of homes are completely destroyed and the city's critical infrastructure is nearly completely destroyed. Ongoing shelling is preventing civilians from evacuating.

The European Union Council plans to finalize a ban on nearly 90% of Russian oil imports by the end of 2022, European Commission President Ursula von Der Leyen said Tuesday at a joint press conference with EU Council President Charles Michel, following Monday's special meeting of the European Council.

Von der Leyen said they will soon return to the issue of the remaining 10% of pipeline oil.

-ABC News' Christine Theodorou

Russia's top diplomat vowed Tuesday to "ensure unhindered passage" of grain shipments from Ukrainian ports to the Mediterranean Sea, if Ukraine removes the mines from its coastal waters.

"Ukrainian representatives should de-mine the coastal waters within Ukraine's territorial sea, which have been mined," Russian Foreign Minister Sergei Lavrov said at a press conference. "If the de-mining problem is resolved -- and we've been bringing the attention of our worried Western counterparts to this problem for many weeks -- then the Russian Navy will ensure unhindered passage of those vessels in high seas to the Mediterranean and further on to their destination points."

Since Russian forces invaded neighboring Ukraine on Feb. 24, the cost of grain, fuel and fertilizer has skyrocketed worldwide, worsening hunger crises. Russia and Ukraine produce a third of the world's supply of wheat and barley, but Kyiv has been unable to ship exports due to Moscow's offensive.

Lavrov, however, blamed the situation on Kyiv and the West.

"For longer than a month now, the Russian side has been taking measures to guarantee the unhindered export of Ukrainian grain by vessels that are currently stranded in Ukrainian ports," Lavrov added. "Therefore, the initiatives concerning food security that have been voiced should be implemented bearing in mind that the Russian side has long guaranteed everything that depends on us. As for the Western countries that have artificially caused numerous problems by closing their ports to Russian vessels and cutting logistical and financial chains, they should certainly think hard whether it's more important for them to advertise themselves taking advantage of food security problems, or take practical steps to resolve this problem. It's for them to choose."

As Russian forces battle for control of a key eastern Ukrainian city, up to 12,000 civilians may be trapped in the crossfire, according to the Norwegian Refugee Council (NRC).

"I am horrified to see Severodonetsk, the thriving city where we had our operational headquarters, become the epicentre of yet another chapter of the brutal war in Ukraine," NRC Secretary-General Jan Egeland said in a statement Tuesday. "We fear that up to 12,000 civilians remain caught in crossfire in the city, without sufficient access to water, food, medicine or electricity. The near-constant bombardment is forcing civilians to seek refuge in bomb shelters and basements, with only few precious opportunities for those trying to escape."

Over the past week, the Oslo-based humanitarian organization has been working with local Ukrainian partners to provide thousands of monthly food and hygiene parcels to civilians remaining in Severodonetsk and the greater Luhansk Oblast, according to Egeland. The city is the last still held by Ukrainian forces in Luhansk Oblast.

"But now the intensified fighting makes aid delivery impossible," he added. "We cannot save lives under the hail of grenades."

NRC has been operational in Ukraine since 2014, serving people affected by conflict in the Luhansk and Donetsk oblasts of eastern Ukraine's war-torn Donbas region.

"Almost one hundred days since the war in Ukraine escalated, we have seen bombs destroy critical infrastructure across the country and reduce entire cities like Severodonetsk to rubble," Egeland said. "More than 14 million men, women and children are displaced within Ukraine or sheltering in other countries with no idea when they will be able to safely return to their homes."

Jalina Porter, the State Department's deputy spokesperson, is responding to the Russian attack at a Ukraine train station that killed at least 50, saying, "We can no longer be surprised by the Kremlin's repugnant disregard for human life."

Five children were among those killed when Russian rockets struck the station in Kramatorsk in Donetsk Oblast on Friday morning, according to Ukraine's state-owned railway company. At least 100 people were injured, according to Donetsk Oblast Gov. Pavlo Kyrylenko.

Russia has denied involvement in the attack, which occurred as "thousands" of civilians fleeing the Russian invasion were at the train station waiting to be taken to "safer regions of Ukraine," according to Kyrylenko.

"Civilians are killed when they stay in their homes, and they're killed when they try to leave," Porter said. "Actions like these demonstrate why Russia did not belong on the U.N. Human Rights Council, and they also reinforce the U.S. assessment that members of Russian forces are committing war crimes in Ukraine."

Porter declined to say if the department considers the train station attack a war crime, saying, "Assessing individual criminal liability in specific cases is the responsibility of courts, as well as other investigatory bodies. But as the secretary, Secretary [of State Antony] Blinken, has said, 'Those responsible for war crimes and other atrocities committed in Ukraine will be held to account.'"

-ABC News' Conor Finnegan