Russian servicemen guard an area of the Zaporizhzhia Nuclear Power Station, southeastern Ukraine. AP

Ukraine's occupied Zaporizhzhia nuclear plant has been cut off from the power grid again, its Russian administrators said on Monday, a potentially dangerous incident that has become more frequent due to shelling.

"Due to a high-tension line being cut, the plant lost its external electricity supply," the Russian administration wrote on Telegram, adding the causes of the outage were being investigated and that back-up diesel generators were keeping it working.

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Another report said, the Zaporizhzhia nuclear power plant was switched to standby and emergency power supply generators, a Russia-installed local official in the Moscow-controlled part of the region said on Monday.

Vladimir Rogov said the plant was "completely" disconnected from external power supply after Ukraine disconnected a power line it controls.

There was no immediate comment from Ukraine, but Anatoliy Kurtev, Zaporizhzhia city council's secretary in Ukraine, said that work was ongoing since early Monday to restore power to the city.

"(The power) partially disappeared in Zaporizhzhia due to an emergency situation at one of the energy facilities," Kurtev said on the Telegram messaging app.