Russian forces are pressing their offensive in eastern Ukraine as they attempt 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’ supply lines to the city and force them to surrender or withdraw.

That would give Russia its first major prize in more than half a year and open the way to the capture of the last remaining urban centres in the 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 Zelensky said on Monday.

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

“Over the past day, our soldiers repelled more than 60 enemy attacks,” the military said early on Tuesday referring to Bakhmut and nearby eastern areas, adding that Ukrainian forces repelled Russian attacks on the villages of Yadhidne and Berkhivka, on the northern approaches to Bakhmut.

Ukrainian military analyst Oleh Zhdanov said Russian forces had driven a wedge between those villages 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.”

Ukrainian soldiers in Donetsk region hunkered in muddy trenches after warmer weather thawed out the frozen ground.

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

Russia, its forces replenished with hundreds of thousands of conscripts, has intensified its attacks along the eastern front but its assaults have come at a high cost, Ukraine and its allies say.

“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.

Russia said its forces had destroyed a Ukrainian ammunition depot near Bakhmut and shot down US-made rockets and Ukrainian drones.

Reuters could not verify the battlefield reports.

The Russian defence ministry said the United States was planning a provocation in Ukraine using toxic chemicals. There was no immediate US response.

US Treasury Secretary Janet Yellen became the latest senior Western official to visit the Ukrainian capital, promising assistance and more measures to isolate Russia after meetings with Mr Zelensky 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,” Ms Yellen told Ukrainian Prime Minister Denys Shmyhal.

Ms Yellen announced the transfer of the first $US1.25 billion ($1.86 billion) from the latest $US9.9 billion tranche of US economic and budget assistance, and visited a school where teacher salaries are reimbursed with US support.

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

Ukraine’s forces have mostly focused on holding defensive positions in recent weeks but are expected to attempt a counteroffensive later in the year with new weapons from the West.

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

“We will be able to completely protect our skies when the aviation taboo is fully lifted in relations with our partners,” Mr Zelensky said in his nightly radio address.

The February 24 first anniversary of Russia’s invasion saw both sides trying to demonstrate resolve for a second year of war.

Russian President Vladimir Putin gave a 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 Mr Biden, who journeyed to Kyiv and gave a landmark speech of his own in Warsaw.