Ukrainian President Volodymyr Zelenskiy said the situation on the front lines in the east of the country was getting tougher and Russia was throwing more and more troops into battle.
The Kremlin has been pushing for a significant battlefield victory after months of setbacks, with Russian forces trying to close grip on the town of Bakhmut and fighting for control of a nearby major supply route for Ukrainian forces.
Russian troops are also trying to capture the coal mining city of Vuhledar, some 120 kilometres southwest of Bakhmut, also in the eastern region of Donetsk.
"I've often had to say the situation at the front is tough, and is getting tougher, and it's that time again... The invader is putting more and more of his forces into breaking down our defences," Zelenskiy said in his nightly video address.
"It is very difficult now in Bakhmut, Vuhledar, Lyman and other directions," he continued.
Earlier in the day, Deputy Defence Minister Hanna Malyar wrote on Telegram that Russian efforts to break the defences in Bakhmut and Lyman had failed.
Lyman, which lies just to the north of Bakhmut, was liberated by Ukrainian forces in October.
On Friday, Zelenskiy vowed that his forces will fight for Bakhmut "for as long as we can," but the situation there is becoming increasingly dire for Ukrainian forces.
The General Staff of Ukraine's Armed Forces has been reporting daily numerous combat clashes in the area and Moscow military bloggers have claimed a number of unverified Russian successes along the frontline.
The fighting around Bakhmut has been costly for Russia in terms of soldiers' lives, the Kremlin admitted.
Russia's independent news outlet Meduza reported in late January that some 40,000 of the 50,000 recruits by the powerful Wagner private military group involved in the campaign there were either dead or missing.
Reuters was not able to independently verify the reports.
Ukrainian military analyst Petro Chernyk said that the high Russian casualty count means Moscow cannot take a break to stop an offensive there to recover as it would ease combat pressure.
"And this would be an excellent condition for our counteroffensive actions," Chernyk told the 24 Kanal Ukrainian television.