"The lives of the people who remain there are in danger," Zelensky told Antonio Guterres by phone, after thanking him for a successful UN and Red Cross-led evacuation this week.

"Everyone is important to us. We ask for your help in saving them."

He called on the UN to "assist in the removal of all the wounded from Azovstal."

The statement came as Ukraine said Wednesday that there were fierce battles with Russian troops at the plant, where Ukrainian soldiers had been holed up for weeks.

Zelensky said that this week's evacuations led by the UN and Red Cross, which he said had brought over 100 people trapped in Azovstal to safety, showed the world "that international organisations can be effective".

He "expressed hope for a successful continuation of the ongoing evacuation operation".

Moscow, which has denied any storming of the plant was taking place, said its forces would cease fire at the plant to open a humanitarian corridor for civilians for three days beginning Thursday.

"The Russian armed forces will from 8 am to 6 pm (0500 GMT to 1500 GMT) on May 5, 6 and 7 open a humanitarian corridor from the territory of the Azovstal metallurgical plant to evacuate civilians," the defence ministry said Wednesday.

Civilians sheltering at the plant will be allowed to travel to Russia or Kyiv-controlled territory, the statement added.