‘It destroys bunkers’: Russia systematically uses thermobaric warheads in Ukraine.

Russia edged closer on Saturday to occupying the entirety of Luhansk, a key province in eastern Ukraine, after its forces entered a critical eastern city still under partial Ukrainian control.

Aided in part by thermobaric warheads, one of the most fearsome conventional weapons available to contemporary armies, the Russian advance in eastern Ukraine highlighted the dividend that Russia has gained by seizing a port on the Black Sea and halting its attempts to capture the Ukrainian capital, Kyiv, and the country’s second-largest city, Kharkiv.

That has allowed the Russian Army to concentrate its forces in a small pocket of eastern Ukraine, where Russian supply lines are less vulnerable; where Russian forces have shored up their control of some newly captured territory; and where Ukrainian officials say their army is now considerably outnumbered and outgunned.

The latest indicator of this dividend came on Saturday, when two senior Ukrainian officials said that Ukrainian and Russian forces were locked in heavy street fighting inside the eastern city of Sievierodonetsk, where Russian soldiers had advanced to within a few blocks of the administrative headquarters. By Saturday morning, the Russians had captured a bus station and a hotel in the city’s northeast and damaged 14 high-rise buildings during at least three rounds of shelling overnight, the head of Luhansk Province’s military administration, Serhiy Haidai, said.

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The last remaining Ukrainian-controlled route into the city was still open, across a bridge spanning a river to the city’s west, said Oleh Hryhory, the provincial police chief. But there was heavy shelling around it, making access to the town extremely dangerous, Mr. Hryhory said.

In his nightly address, President Volodymyr Zelensky of Ukraine said late Saturday that his country’s forces were holding off the Russian assaults on Sievierodonetsk, but acknowledged that they faced “indescribably difficult” conditions.

A railway hub with a peacetime population of about 100,000, Sievierodonetsk is the Ukrainian military’s last significant redoubt in Luhansk Province. While the city is not expected to fall imminently, Russian forces have been making slow but steady gains toward what would be a strategically important victory there.

Its capture would open the way for the Russian forces to set their sights westward to Kramatorsk and Sloviansk, the last major Ukrainian-held cities in the Donbas region, which includes Luhansk and its neighbor Donetsk. Taking them would all but fulfill a goal set forth by President Vladimir V. Putin of Russia on the eve of his invasion of Ukraine in February. Russian-backed separatists seized control in 2014 of parts of Luhansk and Donetsk, and Mr. Putin initially justified his invasion as an attempt to preserve the independence of the two breakaway territories.

Russia’s entry into Sievierodonetsk follows the capture, earlier this week, of Lyman, another strategic city in the region.

In other signs of tightening Russian control in eastern Ukraine, Russian forces reopened a harbor at Mariupol, the Black Sea port that was recently captured by Russia after months of devastating airstrikes and artillery fire that destroyed much of the city. A ship left the port carrying thousands of tons of scrap metal seized from the occupied city, according to Ukrainian officials and a Russian state news agency. It was the first confirmed instance of the port’s use since Russia gained full control of Mariupol.

Associated Press

Ukraine’s president, Volodymyr Zelensky, has repeatedly vowed that Ukraine will retake the entirety of Donbas, rebuffing growing international calls for his country to cede some territory to Moscow in eventual peace talks to end the war.

“Donbas will be Ukrainian,” Mr. Zelensky said in a speech overnight on Friday. For months, Mr. Zelensky has called for heavier weapons to relieve pressure in the Donbas region and turn the tide in the war. United States officials said on Friday that the Biden administration had approved sending long-range multiple launch rocket systems to Ukraine, a move that the Russian foreign minister, Sergey V. Lavrov, has said would be “a serious step toward unacceptable escalation.”

But for now, Ukraine is evacuating civilians from near Sievierodonetsk, in a sign that Ukrainian officials expect further Russian advances in the coming days, amid fears that Russia might encircle the main Ukrainian positions in Donbas.

Out on the highways in Donbas on Saturday, flatbed trucks carrying tanks and trucks towing howitzers rumbled east, suggesting that the Ukrainian military was reinforcing. The Ukrainian Army does not disclose its force numbers but has publicized the arrival of Western weaponry, including long-range American M777 artillery pieces.

Still, military analysts, Ukrainian officials and soldiers on the ground say the Ukrainians remain outgunned by Russia’s far larger arsenal of artillery.

In one engagement on Thursday and Friday in a forest north of the town of Sloviansk, a dozen Ukrainian soldiers were hospitalized with shrapnel wounds after a nearby Ukrainian artillery unit was outgunned by a Russian mortar crew.

Ivor Prickett for The New York Times

Two officers injured in the exchange said Western nations needed to hasten the supply of long-range weapons, including rocket artillery, to even the odds in the battle for Donbas.

“We try to push them back but it doesn’t always work,” said Oleksandr Kolesnikov, a company commander interviewed on a gurney in an ambulance outside a military hospital in Kramatorsk. “We don’t have enough people, enough weapons.”

“You ask how the fighting is going,” Mr. Kolesnikov added. “There was a commander of the company. He was killed. There was another commander. He was killed. A third commander was wounded. I am the fourth.”

The Russian advance has been aided by liberal use of one of its most damaging conventional weapons, the thermobaric warhead, according to Ukrainian military commanders, medics and video from the battlefield.

The weapon, a track-mounted rocket artillery system nicknamed Solntsepek, or the Heat Wave, fires warheads that explode with tremendous force, sending potentially lethal shock waves into bunkers or trenches where soldiers would otherwise be safe.

The missiles scatter a flammable mist or powder that is then ignited and burns in the air. The result is a powerful blast followed by a partial vacuum, as oxygen is sucked from the air as the fuel burns.

“You feel the ground shake,” said Col. Yevhen Shamataliuk, commander of Ukraine’s 95th Brigade, whose soldiers came under fire from the weapon in fighting this month near Izium, a town northwest of Sievierodonetsk.

Finbarr O’Reilly for The New York Times

“It’s a hollow booming sound and the ears ring when it explodes, more than from ordinary artillery,” Colonel Shamataliuk said. “It destroys bunkers. They just collapse over those who are inside. It’s very destructive.”

The United States and other militaries also deploy thermobaric warheads in missiles and rocket-propelled grenades, but analysts say the Russian military’s deployment of the weapon in Ukraine has been among the most systematic uses in recent wars.

But while Russia currently seems to hold the advantage, its advances also come with their own disadvantages. By extending their supply lines, Russian forces themselves become more vulnerable to counterattacks and the logistical complications that plagued Russian maneuvers earlier in the war.

Within Russia, there are also increasing misgivings about whether Russia’s military has the force and resources to continue fighting.

Five opposition deputies in the local legislature of Primorsky Province in Russia’s Far East signed an open letter to Mr. Putin demanding that Russia stop fighting and withdraw its forces. Russia would be better served by using the young men fighting in Ukraine to work in Russia, said the statement read out by Leonid Vasyukevich, a deputy from the nominally opposition Communist Party.

Earlier this week, a diplomat at Russia’s mission to the United Nations in Geneva resigned over the war, the most senior official to leave their post out of opposition to the invasion.

Nicole Tung for The New York Times

And while it supports the war, a grass-roots Russian movement argues that the Kremlin hasn’t done enough to help its soldiers prepare for a major conflict. Led in large part by women, the group is crowdsourcing aid for Russian soldiers, including food and medical supplies.

Within Ukraine, the war has formalized a long-brewing schism within the Orthodox church. Late on Friday, the leaders of the central branch of the Orthodox church in Ukraine made a formal break with the hierarchy in Moscow.

The Council of the Ukrainian Orthodox Church said on Facebook that it was breaking with the Moscow leadership because it disagreed with Patriarch Kirill I, the leader of the Moscow Patriarchate of the Russian Orthodox Church, over his support for the war.

Patriarch Kirill has repeatedly blessed the Russian military forces invading Ukraine. Because he is the church’s spiritual leader in both countries, many of the Ukrainians dying under the onslaught are his followers. He has also avoided condemning attacks on civilians.

The church has been under the wing of the Moscow Patriarchate for centuries, and its departure will markedly decrease the size of the patriarch’s flock because Ukrainians attend church in greater numbers than Russians.

But it is unclear how many of the bishops and parishes in Ukraine will follow the lead of the council, or how many might try to stick with Moscow.

Disputes within the church, which can last for centuries, revolve around complicated questions of doctrine and authority. The church in Ukraine has been wrestling with an internal split since 2014, the year that Russia annexed Crimea and sparked a separatist war in eastern Ukraine.

Ivan Alvarado/Reuters

Reporting was contributed by Carlotta Gall from Bakhmut, Ukraine; Maria Varenikova from Kramatorsk, Ukraine; Anton Troianovski from Istanbul; Erika Solomon from Lviv, Ukraine; and Nadav Gavrielov and Alexandra E. Petri from New York.

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