"Today there are heavy battles on the territory of our fortress, on the territory of Azovstal. Our brave guys are defending this fortress, but it is very difficult, because heavy artillery and tanks are firing all over the fortress; aviation is working, ships have approached and are also firing on the fortress," Boichenko said.

Read here Civilians evacuated from the Azovstal steel plant in Mariupol have arrived in Zaporizhzhia

Speaking on Ukrainian television, Boichenko said there were 30 children trapped at the plant still waiting to be rescued.

"They are waiting for a new negotiation procedure and a new evacuation mission," he said.