Russian forces in a “frenzied push” have seized half of the eastern Ukrainian city of Sievierodonetsk that is key to Moscow’s efforts to quickly complete the capture of the industrial Donbas region, the mayor told The Associated Press on Tuesday.

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

It’s impossible to track civilian casualties amid the round-the-clock shelling, said the mayor, who believes that more than 1,500 residents have died of various causes since Russia’s invasion of Ukraine in late February. Evacuation efforts from Sievierodonetsk have been halted because of shelling.

“Civilians are dying from direct strikes, from fragmentation wounds and under the rubble of destroyed buildings, since most of the inhabitants are hiding in basements and shelters,” Striuk said.