Russian forces were performing secret work at Europe's largest nuclear power plant, according to Ukraine's nuclear energy operator.
It comes as fears rise the Kremlin might intend to make Zaporizhzhia the site of its alleged false flag operation involving a nuclear device.
Russia's defence minister Sergei Shoigu made an unsubstantiated allegation that Ukraine was preparing to launch a so-called dirty bomb.
Shoigu levelled the charge over the weekend in calls to his British, French, Turkish and US counterparts. Britain, France and the United States rejected it right away 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 would not give access to Ukrainian staff running the plant or monitors from the UN'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 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 kilometres (miles) of the
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