SLOVIANSK, Ukraine — Russian forces have seized half of Sievierodonetsk, the eastern Ukrainian city key to Moscow’s efforts to complete the capture of the industrial Donbas region, the city’s mayor Oleksandr Striuk said Tuesday.

“The city is essentially being destroyed ruthlessly block by block,” Striuk said. He described  continued heavy street fighting and artillery barrages that threatened the lives of the estimated 13,000 civilians still sheltering in the ruined city that once was home to more than 100,000.

Serhiy Haidai, governor of the eastern Luhansk region, said Tuesday that “most of Sievierodonetsk” was under Russian control, though he added that fierce fighting continued and the city wasn’t surrounded.

Sievierodonetsk is important to Russian efforts to capture the Donbas before more Western arms arrive to bolster Ukraine’s defense. Moscow-backed separatists have been fighting Ukrainian troops in the region for eight years and held swaths of territory even before the invasion.