Ukraine warned Wednesday that electricity has been entirely cut to its Chernobyl nuclear power plant and radioactive substance could be released because its storage facility cannot cool spent nuclear fuel.

The Chernobyl Exclusion Zone, a 1,000-square-mile restricted area of deserted land surrounding the shuttered plant, was seized by Russian forces just hours after they launched their invasion on Feb. 24. The plant, situated between the Belarus-Ukraine border and the Ukrainian capital, Kyiv, is the site of the world's worst nuclear disaster in 1986.

Ukraine's State Service of Special Communications and Information Protection announced via Twitter on Wednesday that the "Kyiv high-voltage line is currently disconnected due to the damage caused by the occupiers."

"As a result, the Chernobyl station and all nuclear facilities in the Exclusion Zone are without electricity," the agency tweeted.

Ukrainian Foreign Minister Dmytro Kuleba also confirmed the news on Twitter, saying the only electrical grid supplying Chernobyl and all its nuclear facilities occupied by Russian forces "is damaged," causing a loss of power supply.

"I call on the international community to urgently demand Russia to cease fire and allow repair units to restore power supply," Kuleba tweeted.

However, the International Atomic Energy Agency (IAEA) said it "sees no critical impact on safety." The nuclear watchdog of the United Nations tweeted that the "heat load of spent fuel storage pool and volume of cooling water" at Chernobyl is "sufficient for effective heat removal without need for electrical supply."

Some 20,000 spent nuclear fuel assemblies are stored in Chernobyl's storage facility and "need constant cooling," which is only possible if there is electricity, according to Ukraine's State Service of Special Communications and Information Protection.

"If it is not there, the pumps will not cool. As a result, the temperature in the holding pools will increase," the agency tweeted. "After that evaporation will occur, that will lead to nuclear discharge."

Kuleba noted that reserve diesel generators have a 48-hour capacity to power Chernobyl.

"After that, cooling systems of the storage facility for spent nuclear fuel will stop, making radiation leaks imminent," he tweeted. "Putin's barbaric war puts entire Europe in danger. He must stop it immediately!"

Ukraine's State Service of Special Communications and Information Protection warned that "the wind can transfer the radioactive cloud" to other areas of Ukraine as well as Belarus, Russia and elsewhere in Europe. There is also no ventilation inside the Chernobyl storage facility.

"All personnel there will receive a dangerous dose of radiation," the agency tweeted.

Meanwhile, the facility's fire extinguishing system is not functioning and the agency warned of "a huge risk of fire caused by shelling."

"The fight still goes on making it impossible to carry out repairs and restore power," the agency tweeted.

Russia warned Wednesday of "an extremely undesirable and potentially dangerous scenario" if other countries use their airfields to support Ukraine.

When asked by reporters during a daily press briefing to comment on Poland's announcement Tuesday that it's "ready" to "immediately" hand over all its MiG-29 fighter jets "free of charge" to a U.S. air base in Germany to boost Ukraine's fight against Russia, Kremlin spokesman Dmitry Peskov said: "The [Russian] Defense Ministry has already commented on the possibility of using any other airfields for takeoffs of military planes."

"This is an extremely undesirable and potentially dangerous scenario," he added.

More than 2.15 million people have been forced to flee Ukraine since Russian forces invaded on Feb. 24, according to the latest figures from the United Nations refugee agency.

The tally from the U.N. High Commissioner for Refugees (UNHCR) amounts to nearly 5% of Ukraine's population -- which the World Bank counted at 44 million at the end of 2020 -- on the move across borders in just two weeks.

More than half of the refugees are in neighboring Poland, UNHCR figures show.

-ABC News' Zoe Magee

Ukrainian Deputy Prime Minister Iryna Vereshchuk said six humanitarian corridors have been agreed with Russian officials and confirmed with the International Committee of the Red Cross to operate during a temporary cease-fire Wednesday.

According to Vereshchuk, the evacuation routes for civilians are open from towns north of Ukraine's capital, Kyiv, where there has been heavy fighting, as well as from the besieged southeastern port city of Mariupol, where an evacuation failed yesterday. Another route goes from the town of Izium near hard-hit Kharkiv, Ukraine's second-largest city, and another from the eastern city of Volnovakha, where civilians have been trying to evacuate for several days. Another route leads from northeastern city of Energodar, where shelling caused a fire at Ukraine's largest nuclear power plant last week.

Vereshchuk said Russian officials had sent a letter to the Red Cross confirming the routes and a cease-fire for Wednesday from 9 a.m. to 9 p.m. local time. She called on Russia to keep to its commitment and not to violate the cease-fire, as she said it did in Mariupol and Volnovakha on Tuesday.

"We ask Russian forces to commit to their obligations and keep the ceasefire till 9 p.m. as agreed," Vereshchuk said in a statement Wednesday morning.

Vereshchuk noted that an orphanage with 55 children and 26 staff also needs to be evacuated from Vorzel, a town just north of Kyiv.

"The evacuation of them will be done as a separate special operation," she said.

The Russian Ministry of Defense said in a statement Wednesday that it has discussed the interaction on the Ukraine track with the Red Cross

-ABC News' Julia Drozd and Patrick Reevell