Fighting between Russian-speaking separatists and Ukrainian forces has raged in the Donbas for the past eight years.
Analysts say the Kremlin is now focused on seizing the south-eastern region as way of claiming some form of victory in Ukraine.
Some believe Moscow has learned lessons from its failed attempts to capture Kyiv and northern Ukraine, and is bringing in heavy artillery, better frontline command and helicopter support for its battle in the Donbas.
Former tank commander and military analyst Justin Crump told Forces News: "They're going to work best from their hubcap to hubcap, working on a very scripted plan, a very scripted artillery plan, scripted air support and that's more of what we're seeing in the Donbas - they've had time to think about that."
Russian forces are believed to have moved more battalion tactical groups into Ukraine, with analysts stating most of these are being sent to the south and east to boost their chance of taking Mariupol.
The last remaining Ukrainian forces in Mariupol have been holding out inside the vast Azovstal steel works and around the port, despite Moscow's demands for their surrender.
Taking Mariupol would establish a land corridor to Crimea.
It would also free up more Russian units for the battle in the east.
Moscow took Crimea in 2014 but it has long coveted the industrial Russian-speaking regions of Donetsk and Luhansk – known as the Donbas.
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Russian separatists have fought Ukrainian forces there since then, while UK personnel have trained Ukrainian troops.
In February, Russian President Vladimir Putin formally recognised the breakaway regions of Donetsk and Luhansk and amassed forces along the border, and just days later he invaded Ukraine.
Ukrainian President Volodymyr Zelensky told his country last week they should be proud of having resisted the Russian invasion for 50 days.
Mr Putin now refocusing his forces on the Donbas.
Cover image and video thumbnail: Russian MOD.