The Donbas region of Ukraine refers to the old coal and steel producing areas in the east.
The regions, Luhansk and Donetsk, run from outside Mariupol in the south to the northern border.
Russia formally recognizes these breakaway areas as independent of Ukraine.
Russian forces seized more than a third of the territory after Russia annexed Crimea in 2014. Donbas is predominantly Russian-speaking and Putin has repeatedly talked about “liberating” it completely.
Despite sharing a language, these territories are no longer pro-Russian.
Defense specialist Konrad Muzyka, head of Rochan Consulting, said: “Mariupol was one of the most pro-Russian cities in Ukraine and to level it goes beyond my mind.”