All of the reactors at Zaporizhzhia nuclear power plant, where shelling has been ongoing, have been shut down for the first time in the plant’s history, Ukraine's state nuclear regulator Energoatom reported.

Zaporizhzhia -- the largest nuclear power plant in Europe -- has six reactors, two of which are active, according to Ukraine's state nuclear regulator, Energoatom. At 12:12 p.m. local time, the last operating line providing power to the plant was disconnected due to hostilities in the area, and as a result all six reactors were disconnected from the grid for 17 minutes, Energoatom said.

At 12:29 p.m. local time the overhead line was restored and reactors Nos. 5 and 6 start operating again.

At 2:14 p.m. local time, the overhead line was disconnected again, shutting down reactor No. 6 and leaving only reactor No. 5 operating. Work is underway to reconnect No. 6 to the grid.

The International Atomic Energy Agency said the temporary shut down further shows "the urgent need for an IAEA expert mission to travel to the facility."

If external power is lost there's not active circulation of the water that cools the reactor and that could lead to a reactor meltdown. However, the plant "remained connected to a 330 kV line from the nearby thermal power facility that can provide back-up electricity if needed," the IAEA said in a statement. "As a result of the cuts in the 750 kV power line, the ZNPP’s two operating reactor units were disconnected from the electricity grid and their emergency protection systems were triggered, while all safety systems remained operational."

"There was no information immediately available on the direct cause of the power cuts," the IAEA said. "The six-reactor ZNPP normally has four external power lines, but three of them were lost earlier during the conflict. The IAEA remains in close contact with Ukraine and will provide updated information as soon as it becomes available."

Ukrainian President Volodymyr Zelenskyy warned Thursday, "The world must understand what a threat this is: If the diesel generators hadn’t turned on, if the automation and our staff of the plant had not reacted after the blackout, then we would already be forced to overcome the consequences of the radiation accident. Russia has put Ukraine and all Europeans in a situation one step away from a radiation disaster."

He called on the IAEA and other international organizations to act faster, "because every minute the Russian troops stay at the nuclear power plant is a risk of a global radiation disaster."

-ABC News' Christine Theodorou, Fidel Pavlenko and Natalia Shumskaia

President Joe Biden and Ukrainian President Volodymyr Zelenskyy spoke about weapons assistance and concerns over the Zaporizhzhia nuclear plant in their phone call on Thursday, White House press secretary Karine Jean-Pierre said.

Regarding Zaporizhzhia, "We have said Russia should agree to demilitarize the zone around the plant and agree to allow an International Atomic Energy Agency visit as soon as possible," Jean-Pierre said. "This is something that did come up in a conversation."

Zelenskyy tweeted a photo of his phone call with Biden, and said he thanked him "for the unwavering U.S. support for Ukrainian people -- security and financial."

Zelenskyy said he and Biden "discussed Ukraine’s further steps on our path to the victory over the aggressor and importance of holding Russia accountable for war crimes."

Biden also tweeted a photo of the call, and said he congratulated Ukraine on its Independence Day, which was on Wednesday.

"I know it is a bittersweet anniversary, but I made it clear that the United States would continue to support Ukraine and its people as they fight to defend their sovereignty," Biden wrote.

-ABC News' Justin Ryan Gomez

Twenty-one people were killed and another 22 were injured in a Russian missile strike on a train station in Chaplyne, in the central Ukraine region of Dnipropetrovsk, said Kyrylo Tymoshenko, deputy head of the president's office.

-ABC News' Christine Theodorou and Max Uzol

The White House said the U.S. believes Russia could hold "sham referenda" in occupied Ukrainian territory within days or weeks.

White House spokesman John Kirby said last month that Russia was "installing illegitimate proxy officials in the areas of Ukraine that are under its control" who would then arrange "sham referenda" as a precursor to annexation.

Kirby told reporters Wednesday that the U.S. government believes "these referenda could begin in a matter of days or weeks."

"We have information that Russia continues to prepare to hold these sham referenda in Kherson, Zaporizhzhia, the so-called Donetsk and Luhansk People's Republics," Kirby said. "We've also learned that Russian leadership has instructed officials to begin preparing to hold these sham referenda, particularly in Kharkiv, as well."

"We expect Russia to try to manipulate the results of these referenda to falsely claim that the Ukrainian people want to join Russia," he continued. "It will be critical to call out and counter this disinformation in real time."

"Any claim that the Ukrainian people somehow want to join Russia is simply not true," Kirby said, citing polling data. It's clear, he added, that Ukrainians "value and treasure their independence."

President Joe Biden and Ukrainian President Volodymyr Zelenskyy will speak on the phone on Thursday, Kirby said.

-ABC News’ Ben Gittleson