The Kremlin's defense ministry says two Russian commanders have been killed in a battle near the eastern Ukrainian city of Bakhmut.
The ministry announced on Sunday that one of the two commanders died after he received multiple shrapnel wounds as his brigade tried to repel Ukrainian attacks.
It added that Russian forces continued to fight to capture the western parts of Bakhmut.
It also said that all attacks of Ukrainian forces to the north and south of Bakhmut had been repelled.
A US think tank, the Institute for the Study of War, said on Saturday that Ukrainian forces appeared to have retaken part of the Bakhmut area during recent counterattacks.
The British defense ministry said on Sunday that the Russian force is now "mostly poorly trained mobilized reservists and increasingly reliant on antiquated equipment, with many of its units severely under-strength."
The ministry believes the Russians will not be able to "effectively cohere large-scale military effect along the 1,200 km front line under stress."
In western Russia near the border with Ukraine, four Russian aircraft were reportedly shot down on Saturday.
They were believed to be combat jets and helicopters.
Ukraine's air force spokesperson Yurii Ihnat denied his country's involvement in the incident when he spoke to local media on Sunday.
He said Russia had lost a total of five aircraft, not four, including three helicopters.
Ihnat suggested that Russia's own air defenses might have shot down the aircraft by mistake.
Another Ukrainian media outlet reported later that the spokesperson was just responding to a joke.