Russian forces have been trying to close their circle around the small mining city of Bakhmut as rain and an early spring thaw have turned eastern Ukraine’s battlefields to mud, which could hamper both sides as they try to take the initiative in the war.

The spring thaw, known as the “rasputitsa”, has a long history of ruining plans by armies to attack across Ukraine and western Russia because it turns roads into rivers and fields into impenetrable bogs.

In the Donetsk region near the front, Ukrainian soldiers hunkered down 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 front-line rocket launcher battery as he watches a tablet screen for coordinates to fire.

In a trench, cut deeply into 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,” he said.

Russia is trying to encircle Bakhmut and force Ukraine to pull out its garrison. That would give Moscow its first major prize in more than half a year following one of the bloodiest phases of the war so far – a relentless Russian assault that began over frozen ground.

Russia’s forces have been replenished with hundreds of thousands of reservists called up late last year, Russia has intensified its attacks on several locations along the front in the east. Western countries say several of Russia’s assaults have failed at high costs.

But Moscow’s troops have made clear, if slow, progress north and south of Bakhmut as they try to cut off Ukrainian forces inside the ruined city, which once was home to about 75,000 people.

“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 as he described the situation around Bakhmut.

For its part, Moscow said on Monday that it had destroyed a Ukrainian ammunition depot near Bakhmut and shot down US-made rockets and Ukrainian drones. It was not possible to independently confirm the reports.

Two dead in Russia drone attack

Meanwhile, a Russian attack with Iranian-made drones early on Monday killed two people and wounded three in the western Ukrainian city of Khmelnytskyi, its mayor said.

Mayor Oleksandr Symchyshyn said in messages on social media that two rescue workers had died in hospital after the attack with more than a dozen drones.

“Unfortunately, we have another hospital death,” he said in a second statement after earlier reporting the first death. “Doctors failed to save the life of another hero – a rescuer.”

The victims were 21-year-old Vladyslav Dvorak and 31-year-old Sergiy Sevruk, Ukrainian Interior Minister Igor Klymenko said on Facebook.

The two rescue workers responded to the first wave of an attack when “the enemy fired again, deliberately, on our heroes without weapons”, Klymenko said.

The Russian army said it hit a special operations command centre near the city of Khmelnytskyi.

Yellen in Kyiv

In Kyiv, US Treasury Secretary Janet Yellen became the latest senior Western official to visit the Ukrainian capital, promising assistance and more measures to isolate Russia. US President Joe Biden went there a week ago to mark the first anniversary of the war.

“America will stand with Ukraine as long as it takes,” Yellen, flanked by sandbags at the cabinet offices, told Prime Minister Denys Shmyhal.

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

Ukraine’s military forces have mostly focused on holding defensive positions in recent weeks but are expected to mount a counteroffensive using new weapons pledged by the West.

“I really want [victory] to happen this year,” Zelenskyy said in a post on the Telegram messaging app on Monday. “For this we have everything – motivation, confidence, friends, diplomacy.”

The one-year anniversary of the war last week saw efforts from both sides to demonstrate their resolve for a second year of fighting.

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

China, which signalled support for Russia by sending its top diplomat to Moscow last week, has issued a call for peace.

The step has been met sceptically by the West but welcomed in general terms by both Kyiv and Moscow. Washington has said in recent days it worries that China could send weapons to Russia, which Beijing denies.

Russia says the invasion was justified by a security threat posed by Ukraine. Kyiv calls it an unprovoked attempt to subjugate an independent state.

Ukraine’s outnumbered troops last year repelled Russia’s attack on Kyiv and recaptured substantial territory, but Moscow continues to occupy nearly a fifth of Ukraine, which it claims to have annexed.

Tens of thousands of Ukrainian civilians and soldiers on both sides are believed to have been killed.