Ukrainian President Volodymyr Zelenskyy on Wednesday asked the head of the United Nations to help "save" the lives of the remaining wounded Ukrainians trapped underneath the giant Azovstal steel plant in battered Mariupol.

"The lives of the people who remain there are in danger," Zelenskyy 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 the long-awaited evacuations led by the Red Cross and the UN continued from Mariupol and nearby towns.

Deputy Prime Minister Iryna Vereshchuk said on Telegram that a total of 334 people had been evacuated from the area on Wednesday.

"It's another small victory for us," she wrote.

Meanwhile, fierce battles with Russian troops continued to rage at the Azovstal plant, where Ukrainian soldiers had been holed up for weeks.

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 on 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 on Wednesday.

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

In his conversation with Guterres, Zelenskyy said that this week's evacuations showed the world "that international organisations can be effective".

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