About 800 Russian soldiers have been killed in fighting in the past day, according to estimates from Ukraine’s military, as its allies promised more armoured battle vehicles but not the tanks it wants.
Most of the Russian forces died fighting in the eastern Donetsk region, said Ukraine's military.
It said Moscow also lost an aircraft, a helicopter and three tanks.
Ukraine said Russian forces were focused on an offensive in the Bakhmut sector and its attacks in the Avdiivka and Kupiansk sectors were unsuccessful.
It also reported an unspecified number of civilian casualties as a result of Russian air, missile and rocket attacks on the largely ruined, Ukrainian-held city of Bakhmut and two other cities in the Donetsk region, Kostiantynivka and Kurakhove.
Russia denies targeting civilians in what it calls its special military operation in Ukraine.
News agencies on the ground could not independently verify battlefield accounts.
A senior US administration official on Wednesday gave a sobering assessment of fighting in the Donetsk region, especially around Bakhmut.
“The fighting is still quite hot … what we're seeing in Bakhmut we should expect to see elsewhere along the front that there will be continued fighting in the coming months,” the official said.
In his evening video address, President Volodymyr Zelenskyy said Ukrainian troops outside Bakhmut were inflicting numerous losses on their adversaries and Russia was building up its forces in the region.
Ukrainian Deputy Defence Minister Hanna Malyar, citing his ministry's main intelligence directorate, said on the Telegram app that significant losses for Russia meant it would probably have to announce another partial mobilisation in the first quarter of the year.
Yevgeny Balitsky, governor of the Russian-held Zaporizhzhia region, said Ukrainian artillery killed five people and wounded 15 including four emergency workers, Russia's Tass news agency reported.
French President Emmanuel Macron has told Mr Zelenskyy France will send light AMX-10 RC armoured combat vehicles to help the war effort, a French official said on Wednesday, after the two leaders spoke by telephone.
While the official said these would be the first western armoured vehicles delivered to Ukraine, Australia has given Kyiv 90 of its Bushmaster vehicles, an armoured unit hardened against landmines and other threats.
President Joe Biden said later on Wednesday that the US was considering sending Bradley Fighting Vehicles to Ukraine, which is fighting Europe's biggest land conflict since 1945.
Ukraine — in pictures
The Bradley armoured vehicle, which has a powerful gun, has been a US Army staple to carry troops since the mid-1980s. The US Army has thousands of them, and they would give Ukraine more firepower on the battlefield and strengthen its ability in trench warfare.
Mr Biden's decision, however, would fall short of sending the Abrams tanks that Ukraine has sought.
It has repeatedly asked western allies for heavier fighting vehicles such as the Abrams and German-made Leopard tanks.
Mr Zelenskyy thanked Mr Macron for the announcement and said it showed the need for other allies to provide heavier weapons.
“This is something that sends a clear signal to all our partners. There is no rational reason why Ukraine has not yet been supplied with western tanks,” he said.
The US is preparing another package of weapons, which could be announced in the coming days on top of about $21.3 billion in security assistance so far to Ukraine.
The US has increased the capability of the weapons it has sent including shoulder-fired Stinger anti-aircraft missiles, as well as Javelin anti-tank missiles, the Himars rocket system and Nasams surface-to-air missiles.
During a visit by Mr Zelenskyy to Washington last month, the US pledged to send the sophisticated Patriot missile system to repel Russian missile and drone attacks.
Russia launched its “special military operation” on February 24, citing threats to its security and a need to protect Russian speakers. Ukraine and its allies accuse Russia of an unprovoked war to seize territory.
Major Gen Kyrylo Budanov, chief of the Military Intelligence of Ukraine, told ABC News that he expected more strikes “deeper and deeper” inside Russia, without saying whether Ukrainian forces would be responsible.
He said he had been “glad to see” the December 26 attack on Russia's Engels airbase, hundreds of miles from Ukraine's border.
Asked about attacks on Crimea, the peninsula annexed by Russia in 2014, he said: “Crimea is part of Ukraine, it's part of our territory. We can use any weapon on our territory.”
In a signal to the West that Russia will not back down over Ukraine, Russian President Vladimir Putin sent a frigate on Wednesday to the Atlantic Ocean armed with new-generation hypersonic cruise missiles, which can travel at more than five times the speed of sound.