As Vladimir Putin’s war approaches its 100th day, the picture looks decidedly mixed for Ukraine as Russia turns up the heat. Russian forces in a “frenzied push” have seized half of Sievierodonetsk, the eastern Ukrainian city that is key to Moscow’s efforts to complete the capture of the industrial Donbas region, the mayor said Tuesday.

“The city is essentially being destroyed ruthlessly block by block,” Oleksandr Striuk said. He said heavy street fighting and artillery barrages threatened the lives of the estimated 13,000 civilians still sheltering in the ruined city that once was home to more than 100,000.

A Russian airstrike on Sievierodonetsk hit a tank of nitric acid at a chemical factory, causing a huge leak of fumes, according to the governor of the Luhansk region, Serhiy Haidai. He posted a picture of a big cloud hanging over the city and urged residents to stay inside and wear gas masks or improvised ones.

Mr. Haidai said later 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 defense. Moscow-backed separatists have been fighting Ukrainian troops in the region for eight years and held swaths of territory even before the invasion.

President Biden said Tuesday the U.S. will provide Ukraine with the more advanced rocket systems that its leaders have been begging for. In an essay published in the New York Times, Mr. Biden said the rocket systems will enable Ukraine “to more precisely strike key targets.”

U.S. officials, speaking before Mr. Biden’s announcement on condition of anonymity, said Washington will send Ukraine a small number of high-tech, medium-range rocket systems. The rockets could be used both to intercept Russian artillery and to take out Russian positions in towns where fighting is intense, such as Sievierodonetsk.

Sievierodonetsk, which is 90 miles 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.

Mr. Striuk said more than 1,500 residents have died of various causes since Russia’s invasion of Ukraine in late February. Evacuation efforts from Sievierodonetsk have been halted because of shelling.

“Civilians are dying from direct strikes, from fragmentation wounds and under the rubble of destroyed buildings, since most of the inhabitants are hiding in basements and shelters,” Mr. Striuk said.

According to London’s Guardian newspaper, while the U.K.’s Ministry of Defense has made much of Russian losses throughout the war — most recently focusing on the mounting casualties among junior officers — Russian gains in the Donbas appear to tell a different story, for now at least. Increasing anecdotal evidence suggests that Ukraine is suffering heavy casualties in recent fighting in the east, largely from shrapnel, and there are reports of equipment and supply problems, the newspaper reported. It said that Russian forces appear to be preparing for a fresh push farther north amid evidence that they are regrouping near the Ukrainian town of Izium to renew their stalled efforts against Slovyansk.

In his nightly video address, President Zelensky said the situation in the Donbas is “extremely difficult” as Russia has put its army’s “maximum combat power” there.

In the meantime, the AP reported that the European Union’s groundbreaking decision to ban nearly all oil from Russia to punish the country for its invasion of Ukraine is a blow to Moscow’s economy, but its effects may be blunted by rising energy prices and other countries willing to buy Russian oil. 

European Union leaders agreed late Monday to cut Russian oil imports by about 90 percent over the next six months, a dramatic move that was considered unthinkable just months ago.

The 27-country bloc relies on Russia for 25 percent of its oil and 40 percent of its natural gas, and European countries that are even more heavily dependent on Russia had been especially reluctant to act.

The Telegraph reported that member states were split between those desperate to decouple from Kremlin-controlled energy, landlocked nations hopelessly hooked on it, and others somewhere in between.

The EU ban applies to all Russian oil delivered by sea. At Hungary’s insistence, it contains a temporary exemption for oil delivered by the Russian Druzhba pipeline to certain landlocked countries in Central Europe. Hungary’s Viktor Orban compared the embargo to a nuclear bomb being dropped on his country’s economy.

In addition to retaining some European markets, Russia could sell some of the oil previously bound to Europe to China, India, and other customers in Asia, even though it will have to offer discounts, the CEO at consulting firm Macro-Advisory, Chris Weafer, said.

“Now, for the moment, that’s not financially too painful for Russia because global prices are elevated. They’re much higher than last year,” he said. “So even Russia offering a discount means that it’s probably selling its oil for roughly what it sold for last year also.”