KYIV, Ukraine >> 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 made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb. Shoigu leveled the charge over the weekend in calls to his British, French, Turkish and U.S. counterparts. Britain, France and the United States rejected it out of hand as “transparently false.”

Ukraine also dismissed Moscow’s claim as an attempt to distract attention from the Kremlin’s own alleged plans to detonate a dirty bomb, which uses explosives to scatter radioactive waste in an effort 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.

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.”

“We shared it in our telephone conversation with counterparts who have the necessary level of clearance,” he said. “Those who wanted to understand that the threat is serious, they had all the possibilities to understand that. Those who want to reject it as Russian propaganda, they will do it anyway.”

Polyansky said the IAEA can send inspectors to investigate allegations of a “dirty bomb.”

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.”

In a related matter, Russia asked the Security Council to establish a commission to investigate its claims that the United States and Ukraine are violating the convention prohibiting the use of biological weapons at laboratories in Ukraine.

Soon after Russia’s Feb. 24 invasion of Ukraine, its U.N. ambassador, Vassily Nebenzia, claimed that secret American labs in Ukraine were engaged in biological warfare — a charge denied by the U.S. and Ukraine.

Russia has called a Security Council meeting Thursday on Ukraine’s biological laboratories and its allegations.

The Kremlin has insisted that its warning of a purported Ukrainian plan to use a dirty bomb should be taken seriously and criticized Western nations for shrugging it off.

The dismissal of Moscow’s warning is “unacceptable in view of the seriousness of the danger that we have talked about,” Kremlin spokesman Dmitry Peskov said.

Speaking during a conference call with reporters, Peskov added: “We again emphasize the grave danger posed by the plans hatched by the Ukrainians.”

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.”

Dirty bombs don’t have the devastating destruction of a nuclear explosion but could expose broad areas to radioactive contamination.