Staffers at the decommissioned Chernobyl nuclear site in northern Ukraine were being held "hostage" by invading Russian soldiers, White House press secretary Jen Psaki told reporters Thursday.

"We are outraged by credible reports that Russian soldiers are currently holding the staff of the Chernobyl facilities hostage," Psaki said, according to Britain’s BBC.

"This unlawful and dangerous hostage-taking, which could upend the routine civil service efforts required to maintain and protect the nuclear waste facilities, is obviously incredibly alarming and gravely concerning," Psaki continued.

UKRAINE NO LONGER IN CONTROL OF CHERNOBYL SITE, OFFICIAL SAYS

"We condemn it and we request their release."

White House press secretary Jen Psaki speaks at a press briefing at the White House in Washington, Tuesday, Jan. 25, 2022. (AP Photo/Andrew Harnik)

White House press secretary Jen Psaki speaks at a news briefing at the White House in Washington, Tuesday, Jan. 25, 2022.  (Associated Press)

It was not immediately clear how many people were being held captive.

Psaki's comments came after Alyona Shevtsova, an adviser to Ukraine's Ground Forces, wrote on Facebook that Chernobyl staffers were being held captive, The Times of Israel reported.

Earlier Thursday, an adviser to Ukraine President Volodymyr Zelenskyy confirmed that Ukraine had lost control of the Chernobyl site, which was the location of the world’s worst nuclear disaster in 1986.

"After the absolutely senseless attack of the Russians in this direction, it is impossible to say that the Chernobyl nuclear power plant is safe," he said.

A Ukrainian official said Russian shelling hit a radioactive waste repository and an increase in radiation levels was reported, according to The Associated Press. It was not immediately possible for experts to access the site to assess the damage before Russian forces overtook the site, the report said.

Chernobyl nuclear power station's reactor 4, which exploded.

Chernobyl nuclear power station's reactor 4, which exploded. (iStock)

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In a tweet posted Thursday, Ukraine’s foreign minister warned that a repeat of the 1986 disaster could happen with the site now controlled by the Russians.

"In 1986, the world saw the biggest technological disaster in Chornobyl," the Twitter message said. "Russia’s attack on Ukraine may cause another ecological disaster moving its military forces to Chornobyl. If Russia continues the war, Chornobyl can happen again in 2022."

The Associated Press contributed to this report.