Russia's Defence Ministry has announced the demotion of the head of Russian forces in Ukraine after only three months on the job.
Russia's top military officer – the chief of the military's general staff, Gen Valery Gerasimov – was named as the replacement for Gen Sergei Surovikin, who was demoted to deputy.
Britain's Ministry of Defence said putting Gen Gerasimov in charge is "an indicator of the increasing seriousness of the situation Russia is facing, and a clear acknowledgement that the campaign is falling short of Russia's strategic goals".
It added in a tweet that Russian ultra-nationalists and military bloggers critical of Gen Gerasimov are likely to greet the news with "extreme displeasure".
During his short time overseeing the troops in Ukraine, Gen Surovikin was credited with strengthening coordination and reinforcing control. But he also announced a humiliating withdrawal in November from Kherson, the only regional centre Russian forces had captured just weeks after the Kremlin illegally annexed the area.
Gen Gerasimov, meanwhile, was seen as the top architect of the Russian invasion of Ukraine, and critics have blamed him for Moscow's military setbacks.
The Russian Defence Ministry's formal explanation was that expanded military tasks and the need for "closer interaction between branches of the military as well as increasing the quality of supplies and the efficiency of directing groups of forces" prompted the leadership changes.
Battle rages on in Soledar
Soledar, a Ukrainian town just 10 kilometres north of Bakhmut in Donetsk, has seen intense fighting in recent days.
Ukrainian officials denied Russian claims that Soledar had fallen but the Wagner Group's owner repeated the assertion of a breakthrough late on Wednesday.
However, the owner of the mercenary Wagner Group, Yevgeny Prigozhin wrote on his Russian social media platform: "Once again I want to confirm the complete liberation and cleansing of the territory of Soledar from units of the Ukrainian army.
"Civilians were withdrawn. Ukrainian units that did not want to surrender were destroyed."
He claimed about 500 people were killed and that “the whole city is littered with the corpses of Ukrainian soldiers”.
Ukraine's military said late on Wednesday Russian forces had suffered "huge losses" in the Soledar fighting. The AP was unable independently to verify either side's claims.
Kremlin spokesman Dmitry Peskov stopped short of declaring the town's capture, telling reporters Russian forces had achieved "positive dynamics in advancing" in Soledar.
"Let's not rush, and wait for official statements," he added.