KYIV, Oct 25 - Ukraine’s nuclear energy operator said Tuesday that Russian forces were performing secret work at Europe’s largest nuclear power plant, activity that could shed light on Russia’s claims that the Ukrainian military is preparing a “provocation” involving a radioactive device.

Russian Defense Minister Sergei Shoigu earlier this week made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb, while Ukraine dismissed Moscow’s claim as an attempt to distract attention from the Kremlin’s own alleged plans to detonate a dirty bomb to sow terror.

Energoatom, the Ukrainian state enterprise that operates the country’s four nuclear power plants, said Russian forces have carried out secret construction work over the last week at the occupied Zaporizhzhia Nuclear Power Plant in Ukraine, The Associated Press reported.

Russian officers controlling the area won’t give access to Ukrainian staff running the plant or monitors from the U.N.’s atomic energy watchdog that would allow them to see what the Russians are doing, Energoatom said Tuesday in a statement.

Energoatom said it “assumes” the Russians “are preparing a terrorist act using nuclear materials and radioactive waste stored at” the plant. It said there were 174 containers at the plant’s dry spent fuel storage facility, each of them containing 24 assemblies of spent nuclear fuel.

“Destruction of these containers as a result of explosion will lead to a radiation accident and radiation contamination of several hundred square kilometers (miles) of the adjacent territory,” the company said.

It called on the International Atomic Energy Agency to assess what was going on.

The U.N. Security Council held closed-door consultations Tuesday about the dirty-bomb allegations at Russia’s request.

Russia’s U.N. Ambassador Vassily Nebenzia sent a five-page letter to council members before the meeting claiming that according to the Russian Ministry of Defense, Ukraine’s Institute for Nuclear Research of the National Academy of Sciences in Kyiv and Vostochniy Mining and Processing Plant “have received direct orders from (President Volodymyr) Zelenskyy’s regime to develop such a dirty bomb” and “the works are at their concluding stage.”

Nebenzia said the ministry also received word that this work “may be carried out with the support of the Western countries.” And he warned that the authorities in Kyiv and their Western backers “will bear full responsibility for all the consequences” of using a “dirty bomb,” which Russia will regard as “an act of nuclear terrorism.”

Russia’s deputy U.N. ambassador Dmitry Polyansky was asked by reporters after the council meeting what evidence Russia has that Zelenskyy gave orders to develop a “dirty bomb.” He replied, “it is intelligence information.”

Britain’s deputy U.N. ambassador James Kariuki told reporters after the meeting that “we’ve seen and heard no new evidence” and the U.K., France and the U.S. made clear “this is a transparently false allegation” and “pure Russian misinformation.” He said, “Ukraine has been clear it’s got nothing to hide” and “IAEA inspectors are on the way.”

At the White House, U.S. President Joe Biden was asked Tuesday if Russia is preparing to deploy a tactical nuclear weapon after making its claims that Ukraine will use a dirty bomb.

“I spent a lot of time today talking about that,” Biden told reporters.

The president was also asked whether the claims about a Ukrainian dirty bomb amounted to a false-flag operation.

“Let me just say, Russia would be making an incredibly serious mistake if it were to use a tactical nuclear weapon,” Biden said. “I’m not guaranteeing you that it’s a false-flag operation yet ... but it would be a serious, serious mistake.” (ap/ez)