Ukraine's energy minister has said that a "technical fault" in Europe's largest nuclear power plant has cut production but poses no danger.
Volodymyr Demchyshyn said on Wednesday that one of the reactors at the Zaporizhzhya plant in the south of Ukraine had automatically shut down.
There was no problem with the reactor itself and the problem would be fixed by the weekend, he added.
Zaporizhzhya produces more than one-fifth of Ukraine's electricity.
The operator of the nuclear plant, Energoatom, had reported a problem with the power plant on Friday.
But the details were not fully disclosed until a cabinet meeting on Wednesday when Prime Minister Arseniy Yatsenyuk asked the energy minister to report on what had happened.
Mr Demchyshyn said that damage to a transformer in a power-generating unit had triggered an automatic shutdown of one of the six 1,000 MW reactors.
He added that there was "no threat" to the safety of the reactor.
The French Institute for Radiological Protection and Nuclear Safety (IRSN) told Reuters that their sensors had not detected anything unusual in Ukraine.
Michel Chouha, IRSN's special representative for Central and Eastern Europe, said: "If there was an accident, we would know."
The drop in electricity production has left several dozen towns and villages without electricity, Russian media reported.
Ukraine's energy industry is already under pressure because of a fuel shortage.
Mr Demchyshyn said that Ukraine will probably have to import power from Russia this winter.
He called it a "necessary step" despite how "politically problematic it might be".
Last year Ukraine was self-sufficient in electricity but the conflict with pro-Russian rebels in the east of the country has severely hit coal production.
Ukraine has already agreed a $4.6bn package of gas supplies from Russia, ending months of wrangling with Moscow over prices.