Russia began storming the Azovstal steel mill in Mariupol on Tuesday with civilians still trapped inside – as others who had been evacuated from the bombed-out plant reached safer territory and spoke of the “hell” of constant shelling.

The site had become last pocket of resistance in the Ukrainian port city, whose capture would allow Russia to establish a land corridor to the Crimean Peninsula and free up troops for fighting elsewhere in the Donbas.

It came as Russian rockets pounded other parts of eastern and southern Ukraine and targeted dumps of advanced western military hardware, while Boris Johnson invoked Winston Churchill in a speech promising that Ukraine’s western-backed government would defeat Russia’s invasion and secure its freedom.

Russian troops shelled a chemical plant in the eastern city of Avdiivka, killing at least 10 people, and explosions were also heard in Lviv, near the Polish border. The strikes damaged two power substations, knocking out electricity in parts of the city. Lviv has been a gateway for Nato-supplied weapons and a haven for those fleeing the fighting in the east.

The first – and so far only – civilians evacuated from the shattered Azovstal steelworks came during a brief ceasefire over the weekend, in an operation overseen by the UN and the Red Cross.

Osnat Lubrani, the UN humanitarian coordinator for Ukraine, said that thanks to the evacuation effort, “101 women, men, children and older persons could finally leave the bunkers below the Azovstal steelworks and see the daylight after two months.”

One evacuee said she went to sleep there every night afraid she wouldn’t wake up. “You can’t imagine how scary it is when you sit in the shelter, in a wet and damp basement which is bouncing, shaking,” Yelena Tsybulchenko said upon arriving in the Ukrainian-controlled city of Zaporizhzhia in a convoy of buses and ambulances.

She added: “We were praying to God that missiles fly over our shelter, because if it hit the shelter, all of us would be done.”

The news for those left behind was more grim. Ukrainian commanders said Russian forces backed by tanks began storming the sprawling plant, which includes a maze of tunnels and bunkers spread out over four square miles.

How many Ukrainian fighters were holed up inside was unclear, but the Russians put the number at about 2,000 in recent weeks, and 500 were reported to be wounded. A few hundred civilians also remained there, Ukrainian deputy prime minister Iryna Vereshchuk said.