Ukrainian President Volodymyr Zelenskyy has said the situation on the front lines in the east of the country was getting tougher with Russia 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 strengthen the 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, 120km south-west of Bakhmut 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," Mr Zelenskyy 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, Mr Zelenskyy 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.
The head of Russia's private Wagner militia said on Sunday that fierce fighting was ongoing in the northern parts of Bakhmut.
Yevgeniy Prigozhin, the founder and head of the Wagner group, said his soldiers were "fighting for every street, every house, every stairwell" against Ukrainian forces who were not retreating.
Russia's independent news outlet Meduza reported in late January that 40,000 of the 50,000 recruits by the powerful Wagner private military group involved in the campaign there were either dead or missing.
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," Mr Chernyk told the 24 Kanal Ukrainian TV channel.
Helping to arm Ukraine so it can defend itself against Russia is the swiftest path to achieving peace, British Foreign Secretary James Cleverly said in an article published on Sunday.
Cleverly was writing in a Maltese newspaper ahead of a visit on Tuesday to the Mediterranean island, which assumed the presidency of the UN Security Council at the start of February.
"Like all authoritarian rulers, (Russian President Vladimir) Putin responds only to strength in his opponents," Mr Cleverly wrote in the Times of Malta.
He added that he was delighted that Germany and the United States had joined Britain in agreeing to send tanks to Ukraine.
"Giving the Ukrainians the tools they need to finish the job is the swiftest — indeed the only — path to peace," he wrote.