Iryna Vereshchuk, the Ukrainian deputy prime minister, wrote on the Telegram messaging app: “Today, the Russian side once again announced the existence of a corridor for civilians to leave Azovstal. This could be believed if the Russians had not destroyed humanitarian corridors many times before.

“It is important to understand that a humanitarian corridor opens by the agreement of both sides. A corridor announced unilaterally does not provide security, and therefore is not a humanitarian corridor.” 

Ms Vereshchuk added that Ukraine had “appealed” to Antonio Guterres, the UN secretary general, “to be the initiator and guarantor of the humanitarian corridor from Azovstal for civilians”.

New video footage posted on social media has revealed how the tunnels beneath Azovstal are deep and meandering, making them an ideal location for defending against Russian invaders.

The clip, posted by the Visegrad 24 news website, was accompanied by eerie piano music as the camera moved around a series of maze-like corridors.

Civilians hiding beneath the plant face dire conditions. Recent videos from the tunnels appeared to show that babies were having to wear improvised nappies made from plastic bags.