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 all the way to the northern border.
Russia formally recognises these breakaway areas as independent from Ukraine.
Russian forces seized more than a third of the area after Russia annexed Crimea in 2014. Donbas is predominantly Russian-speaking and Putin has repeatedly spoken about “liberating” it completely.
Despite sharing a language, these areas are no longer pro-Russian.
Defence specialist Konrad Muzyka, head of Rochan Consulting, said: “Mariupol was one of the most pro-Russian cities in Ukraine and to level it is beyond my comprehension.”