Ukraine's President Volodymyr Zelenskyy appealed to the head of the United Nations on Wednesday for assistance in rescuing the remaining wounded Ukrainians trapped beneath the massive Azovstal steel factory in Mariupol.

"The lives of the people who remain there are in danger," Mr. Zelenskyy warned UN Secretary-General Antonio Guterres by phone after expressing gratitude for this week's successful evacuation.

"Every individual is significant to us. We enlist your assistance in rescuing them."

He pleaded with the United Nations to "assist in the removal of all the wounded from Azovstal."

Ukraine claimed that heavy clashes occurred with Russian troops at the facility, where the Ukrainian military had been holed up for weeks alongside residents.

Mr. Zelenskyy stated that this week's evacuations, led by the United Nations and the Red Cross, rescued over 100 persons trapped in the Azovstal plant, demonstrating "that international organizations can be effective."

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

Moscow, which has denied seizing the plant, said its soldiers would suspend fire there for three days beginning Thursday to provide a humanitarian corridor for people.

"The Russian armed forces will, from 8am to 6pm on May 5, 6 and 7, open a humanitarian corridor from the territory of the Azovstal metallurgical plant to evacuate civilians," the defense ministry stated on Wednesday.

The ministry announced that civilians seeking refuge at the factory would be permitted to travel to Russia or Ukraine-controlled territory.