Russian forces pressed their offensive in eastern Ukraine as they attempted to encircle the small mining city of Bakhmut, the scene of the toughest fighting in battlefields saturated by rain and an early spring thaw.

Russia is trying to cut the Ukrainian defenders’ vulnerable supply lines into the city and force them to surrender or withdraw. That would give Moscow its first major prize in more than half a year, and open the way to the capture of the last remaining urban centres in Donetsk region.

“The enemy is constantly destroying everything that can be used to protect our positions for fortification and defence. Our soldiers defending the area around Bakhmut are true heroes,” Ukrainian President Volodymyr Zelenskyy said on Monday (27 February).

Ukraine’s general staff said Russia had strengthened its forces in the Bakhmut area and was shelling settlements around the city.

Ukrainian soldiers in Donetsk region hunkered in muddy trenches after suddenly warmer weather softened the frozen ground.

“Both sides stay in their positions, because as you see, spring means mud. Thus, it is impossible to move forward,” said Mykola, 59, commander of a Ukrainian frontline rocket launcher battery, watching a tablet screen for coordinates to fire.

The spring thaw, known as the rasputitsa, has a history of ruining plans by armies to attack across the soil of Ukraine and western Russia, turning roads into rivers and fields into impenetrable bogs.

Reuters saw several military vehicles stuck in mud. In a trench, cut deeply out of the ground in a zigzag pattern, Volodymyr, a 25-year-old platoon commander, said his men were prepared to operate in any weather.

“When we’re given a target that means we have to destroy it.”

Its forces replenished with hundreds of thousands of conscripts called up late last year, Russia has intensified its attacks along the front in the east.

Western countries say several of Russia’s assaults on Bakhmut have failed at high cost.

“Vicious battles are going on there. The command is doing everything it can to stop the enemy from advancing through our territory,” Serhiy Cherevatyi, a spokesperson for Ukraine’s eastern military command, told Ukrainian television.

For its part, Moscow claimed to have destroyed a Ukrainian ammunition depot near Bakhmut and shot down US-made rockets and Ukrainian drones.

Reuters could not verify the battlefield reports.

Ukrainian military analyst Oleh Zhdanov said Russian forces had driven a wedge between the villages of Yadhidne and Berkhivka, on the northern approaches to Bakhmut, as they tried to cut the road west to Chasiv Yar.

“The southern part of Bakhmut is the only area which can be described as under Ukrainian control. In all other districts, the situation is unpredictable,” he said in a video commentary, adding: “It is impossible to say where the front line lies.”

Yellen in Kyiv

US Treasury Secretary Janet Yellen became the latest senior Western official to visit the Ukrainian capital Kyiv, promising assistance and more measures to isolate Russia after meetings with Zelenskiy and other officials.

Her boss, President Joe Biden, went there a week ago to mark the first anniversary of Russia’s invasion of Ukraine.

“America will stand with Ukraine as long as it takes,” Yellen, flanked by sandbags at the Cabinet ministers’ office, told Ukrainian Prime Minister Denys Shmyhal on Monday.

Yellen announced the transfer of the first $1.25 billion from the latest, $9.9 billion tranche of economic and budget assistance from Washington, and visited a school where teacher salaries are reimbursed by US budgetary support.

She also backed completion of a fully financed program for Ukraine with the International Monetary Fund by the end of March.

Ukraine’s military forces have mostly focused on holding defensive positions in recent weeks, but are expected to attempt a counter-offensive later this year using new weapons pledged by the West.

Zelenskyy pushed again for F-16 fighter jets that his Western allies have been reluctant to provide.

“Our pilots and anti-aircraft units, and other experts of our air force are doing a great job,” Zelenskyy said in his nightly radio address. “But we will be able to completely protect our skies when the aviation taboo is fully lifted in relations with our partners.”

The 24 February war anniversary saw both sides trying to demonstrate their resolve for a second year of fighting.

Russian President Vladimir Putin gave a major speech in which he abandoned the last remaining nuclear arms control treaty with the United States but announced no major initiatives to change the war’s course.

He was upstaged by Biden, who journeyed to Kyiv and gave a landmark speech of his own in Warsaw.

Ukraine’s outnumbered troops repelled Russia’s attack aimed at taking the capital early in the war and later recaptured substantial territory. Russia still occupies nearly a fifth of Ukraine which it claims to have annexed.