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

Queen Elizabeth released a statement Wednesday marking Ukrainian Independence Day.

"It gives me great pleasure to send Your Excellency and the people of Ukraine my warmest greetings on the celebration of your Independence Day," she said. "In this most challenging year, I hope that today will be a time for the Ukrainian people, both in Ukraine and around the world, to celebrate their culture, history and identity. May we look to better times in the future."