KHARKIV, Ukraine—Ukrainian officials accused Russia of burying thousands of civilians in mass graves outside Mariupol, a city now mostly under Russian control, as a senior Russian military official said Moscow’s territorial goals extend well beyond Ukraine’s eastern Donbas region.

Moscow stopped considering Donbas as part of Ukraine—including areas controlled by Kyiv—when President Vladimir Putin in February recognized the independence of the so-called Donetsk and Luhansk people’s republics, statelets created with Moscow’s help in part of the region in 2014.

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