Russia warned on Thursday of the risk of a man-made nuclear disaster at the Zaporizhzhia nuclear power plant and accused Ukraine of planning a "provocation" there on Friday during a visit by UN Secretary-General Antonio Guterres.
The plant, near front lines, has come under fire repeatedly in recent weeks, with both Ukraine and Russia blaming each other for the shelling.
The Russian defense ministry accused Ukraine of trying to stage a "minor accident" at the plant in southern Ukraine in order to blame Russia.
Reuters could not verify Russia's assertion.
The Zaporizhzhia nuclear reactor complex, the largest in Europe, was captured by Russian forces soon after tens of thousands of troops entered Ukraine on February 24 in what Russia calls a "special military operation."
Russian defense ministry spokesman Igor Konashenkov told reporters that Moscow was taking measures to ensure safety at the complex and denied it had deployed heavy weapons in and around the plant.
Plant may be shut down
The ministry said the plant may be shut down if Ukrainian forces continued shelling it.
In a briefing, Igor Kirillov, head of Russia's radioactive, chemical and biological defense forces, said the plant's back-up support systems had been damaged as a result of shelling.
Kirillov presented a slide, showing that in the event of an accident at the plant, radioactive material would cover Germany, Poland and Slovakia.
Guterres, who is set to meet Ukrainian President Volodymyr Zelensky later on Thursday, has called for a halt to all fighting near the plant.
Yevgeny Balitsky, the head of the Russian-installed administration in the Zaporizhzhia region, said earlier there was a risk that shelling could damage the cooling system of the reactor complex.
Russia says Ukrainian forces are recklessly firing at the plant.
Ukraine says Russia is deliberately using the plant as a base to launch attacks against its forces and Russian troops have struck it themselves to blame Ukraine for any subsequent power cuts.