Ukrainian service members are seen next an infantry fighting vehicle near the frontline town of Bakhmut, amid Russia's attack on Ukraine, in Donetsk region, Ukraine. Reuters

"Despite significant losses, the enemy threw in the most prepared assault units of Wagner, who are trying to break through the defences of our troops and surround the city," Syrskyi was quoted as saying on the Ukraine's Media Military Centre Telegram messaging platform.

READ MORE

US warns China against providing lethal aid for Russia's war in Ukraine

Saudi Arabia signs $400 million agreement for Ukraine aid

Earlier, 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' 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.

A Ukrainian serviceman prepares to operate a drone as the sounds of shelling continue in Bakhmut. AFP

"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 Zelenskiy 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.

This photograph shows a bathtub in the ruble of a destroyed and burnt residential building, in Bakhmut on Monday. AFP

"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.

"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 Ukraine and western Russia, turning roads into rivers and fields into quagmires.

A local resident walks down a street as the sounds of shelling continue in Bakhmut on Monday. AFP

Reuters saw several military vehicles stuck in mud. In a zigzag trench, 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, 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.