By Andrew E. Kramer, Matthew Mpoke Bigg, Neil Macfarquhar and Patrick Kingsley

KYIV – Russia edged closer on Saturday (28) 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 Saturday, when two senior Ukrainian officials said 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, said the head of Luhansk province’s military administration, Serhiy Haidai.

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, provincial police chief. But there was heavy shelling around it, making access to the town extremely dangerous, Hryhory said.

In his nightly address, Ukrainian President Volodymyr Zelenskyy 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. Although 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 Slovyansk, the last major Ukrainian-held cities in the Donbas region, which includes Luhansk and its neighbour Donetsk. Taking them would all but fulfil a goal set forth by Russian President Vladimir Putin on the eve of his invasion of Ukraine in February. Russian-backed separatists seized control in 2014 of parts of Luhansk and Donetsk, and 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, this past week, of Lyman, another strategic city in the region.

In other signs of tightening Russian control in eastern Ukraine, Russian forces reopened a harbour 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.

Zelenskyy 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,” Zelenskyy said in a speech overnight on Friday (27). For months, Zelenskyy has called for heavier weapons to relieve pressure in the Donbas region and turn the tide in the war. United States officials said Friday that the Biden administration had approved sending long-range multiple launch rocket systems to Ukraine, a move that the Russian Foreign Minister Sergey 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 Thursday (26) and Friday in a forest north of the town of Slovyansk, a dozen Ukrainian soldiers were hospitalized with shrapnel wounds after a nearby Ukrainian artillery unit was outgunned by a Russian mortar crew.

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,” 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 Izyum, a town northwest of Sievierodonetsk.

“It’s a hollow booming sound, and the ears ring when it explodes, more than from ordinary artillery,” 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.

Although 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 manoeuvres 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 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.

This past 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.

And although it supports the war, a grassroots 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.

-New York Times