During a meeting in Lviv on Thursday with United Nations Secretary-General Antonio Guterres, Ukrainian President Volodymyr Zelenskyy called on the U.N. to ensure the demilitarization and "complete liberation" of the Zaporizhzhya nuclear power plant from Russian forces, according to a statement from his office.

The two "agreed upon the parameters" of a possible visit to the plant by the International Atomic Energy Agency, the U.N.'s nuclear watchdog, Zelenskyy's office said.

Russia has claimed a demilitarized zone around the plant, the largest nuclear power plant in Europe, would make it more vulnerable.

During their meeting in Lviv, Zelenskyy also called for a U.N. fact-finding mission to head to Olenivka, where dozens of Ukrainian prisoners of war were killed in an explosion late last month.

-ABC News' Christine Theodorou

The international calls and proposals for Russia to create a demilitarized zone around the Zaporizhzhia nuclear power plant in southeastern Ukraine are "unacceptable," according to Ivan Nechayev, deputy director of the Russian Foreign Ministry's Information and Press Department.

"Their implementation will make the plant even more vulnerable," Nechayev said at a press briefing on Thursday.

Moscow is expecting experts from the International Atomic Energy Agency, the nuclear watchdog of the United Nations, to visit the Zaporizhzhia plant "in the near future," according to Nechayev.

The secretary-generals of the U.N. and the IAEA have called for the establishment of a demilitarized zone around the Russian-occupied Zaporizhzhia plant, which is the largest nuclear power plant in Europe.

Shortly after invading neighboring Ukraine on Feb. 24, Russian troops stormed the Zaporizhzhia plant, near the town of Enerhodar, on the banks of the Dnipro River in the country's southeast. The Ukrainian workers have been left in place to keep the plant operating, as it supplies electricity across the war-torn nation. However, heavy fighting around the site has fueled fears of a catastrophe, like what happened at the Chernobyl nuclear power plant in northern Ukraine over 36 years ago.

A Ukrainian firefighter who responded to the Russian missile attacks in Kharkiv overnight told ABC News that the scale of the blasts was "one of the biggest" he's ever seen.

One of the rockets struck a large apartment block on Wednesday night, killing at least nine people and injuring another 16, according to Ukrainian authorities.

"It went through all four floors and hit the ground and almost blew up everything," the firefighter, Roman Kachanov, told ABC News during an interview on Thursday. "All the buildings around were without windows."

"There was a dormitory, and the building was almost completely ruined," he added. "There was a playground that was smashed like a big titan blew it up."

Kachanov is among the rescue workers searching for survivors amid the smoldering rubble.

"I’ve seen three bodies on the floor covered by objects," he said. "We tried to extract them and while we tried, the other wall started to fall and we had to run away as fast as we can."

Kachanov said another missile hit the city before dawn Thursday, not far from where he and his team were working. He said the blast "was very loud" and "sounded close."

"Everyone had to lay down," he recalled. "The team had to split -- fire truck had to leave to go to that other fire."

-ABC News' Britt Clennett, Dragana Java, Natalya Kushnir and Sohel Uddin

At least seven people are dead and another 13 injured by strikes on a large apartment block in Kharkiv, officials said.

Based on recovered shrapnel, authorities determined an Iskander-M missile system was used in the strike, said Ivan Sokol, Ukraine's director of the regional Department of Civil Defense.

>Search and rescue efforts are ongoing at the three-story residential building, the State Emergency Service of Ukraine said.

-ABC News' Tatiana Rymarenko