Russian forces have pushed deeper into Ukraine’s besieged and battered port city of Mariupol where heavy fighting shut down a major steel plant and local authorities pleaded for more help.

The fall of Mariupol, the scene of some of the war’s worst suffering, would mark a crucial battlefield advance for the Russians, who are largely bogged down outside key cities more than three weeks into the biggest land invasion in Europe since World War II.

“Children, elderly people are dying. The city is destroyed and it is wiped off the face of the earth,” Mariupol police officer Michail Vershnin said on Saturday from a rubble-strewn street in a video addressed to Western leaders.

Russian forces have already cut the city off from the Sea of Azov, and its fall would link Crimea, which Russia annexed in 2014, to territories controlled by Moscow-backed separatists in the east.