The United States said it's sending two Patriot anti-missile batteries stationed in Europe to Poland as a "defensive deployment" at the request of the Polish government.

While testifying before Congress Tuesday on Russia's invasion of Ukraine, U.S. Under Secretary of State for Political Affairs Victoria Nuland said there were discussions underway with the Polish government about a possible deployment of Patriot batteries. The U.S. military's European Command (EUCOM) later confirmed that two batteries already in Europe would be deployed to Poland.

"At the direction of the Secretary of Defense and at the invitation of our Polish allies, General Wolters, Commander of U.S. European Command, has directed U.S. Army Europe and Africa to reposition two Patriot Batteries to Poland," EUCOM spokesman Capt. Adam Miller said in a statement Tuesday. "This defensive deployment is being conducted proactively to counter any potential threat to U.S. and Allied forces and NATO territory. This is a prudent force protection measure that underpins our commitment to Article Five and will in no way support any offensive operations. Every step we take is intended to deter aggression and reassure our Allies."

The move came hours after the U.S. dismissed Poland's offer to transfer all of its MiG-29 fighter jets to a U.S. air base in Germany to boost Ukraine's fight against Russia, with Pentagon press secretary John Kirby saying in a statement Tuesday that "we do not believe Poland's proposal is a tenable one."

Poland is expecting delivery later this year of two Patriot batteries it had bought in 2018. The air defense systems are intended to shoot down incoming missiles, so their deployment to Poland means there are concerns about dealing with any incoming missile fire into the country, which shares a 330-mile border with Ukraine. It was unclear exactly where in Poland the Patriot batteries would be placed.

-ABC News' Luis Martinez

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