Control of Donbas would allow Moscow to create a southern corridor to the occupied Crimean peninsula. Ahead of the widely anticipated advance, Ukrainian authorities had urged people in Donbas to flee west to escape
Russia has launched a major offensive into eastern Ukraine, authorities in Kyiv said, opening a new phase of its invasion after being thwarted in efforts to capture the capital.
The United States is due to hold a video meeting with allies Tuesday to discuss the conflict in Ukraine, where Moscow's military campaign has refocused on the eastern region of Donbas, partly controlled by pro-Kremlin separatists since 2014.
"We can now confirm that Russian troops have begun the battle for the Donbas, which they have been preparing for a long time," Ukrainian President Volodymyr Zelensky said on Telegram late Monday.
"No matter how many Russian soldiers are brought here, we will fight. We will defend ourselves."