By Leonardo Benassatto
DONETSK PROVINCE, Ukraine (Reuters) -Russian forces were trying to close their circle around the small mining city of Bakhmut on Monday, while rain and an early spring thaw turned eastern Ukraine's battlefields to mud which could hamper both sides as they try to take the initiative.
The spring thaw, known as the rasputitsa, has a long 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.
In the Donetsk region near the front, Ukrainian soldiers 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.
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."
Russia is trying to encircle Bakhmut, forcing 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.
Its forces 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 cost.
But Moscow's troops have made clear, if slow, progress north and south of Bakhmut, attempting to cut off Ukrainian forces inside the ruined city, which once held around 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, describing the situation around Bakhmut.
For it's part, Moscow claimed to have destroyed a Ukrainian ammunition depot near Bakhmut and shot down U.S.-made rockets and Ukrainian drones. Reuters could not verify the battlefield reports.
President Volodymyr Zelenskiy on Monday described the situation around Bakhmut as increasingly difficult, saying the Russian forces are "constantly destroying everything that can be used to protect our positions for fortification and defence."
YELLEN IN KYIV
U.S. 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, which Moscow describes as a "special military operation" to protect Russian security.
Ukraine calls the war an unprovoked attempt to subjugate an independent state.
"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.
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 U.S. 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.
"I really want (victory) to happen this year. For this we have everything – motivation, confidence, friends, diplomacy," Zelenskiy said on the Telegram messaging app.
One thing missing are F-16 fighter jets his country has been seeking from reluctant Western allies.
"Our pilots and anti-aircraft units, and other experts of our air force are doing a great job," Zelenskiy 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 Feb. 24 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.
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.
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.
(Reporting by Reuters bureaux; Writing by Peter Graff and Doina Chiacu; Editing by Mike Collett-White, Christina Fincher and Grant McCool)