Staff at Ukraine's Zaporizhzhia nuclear plant occupied by Russian soldiers were on Friday working to reconnect its reactors to the national power grid, the state energy operator said.
The plant -- Europe's largest facility -- was cut off from Ukraine's power network for the first time in its history on Thursday due to "actions of the invaders", Energoatom said.
Zaporizhzhia has been cause for mounting concern since it was seized by Russian troops in the opening weeks of the war.
In recent weeks, Kyiv and Moscow have traded blame for rocket strikes around the facility in the southern Ukrainian city of Energodar.
Ukrainian President Volodymyr Zelensky said late Thursday the cut-off was caused by Russian shelling of the last active power line linking the plant to the network.
"Russia has put Ukrainians as well as all Europeans one step away from radiation disaster," he said in his nightly address.
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Early Friday, Energoatom said on Telegram that all reactors remain "disconnected from the electrical grid" as of 9 am local time (0600 GMT).
However a severed power line -- the cause of the outage -- has been "restored" and "work is ongoing to prepare the connection" of two of the plant's six reactors.
The International Atomic Energy Agency (IAEA) has previously said the situation at the plant is "highly volatile" and "underlines the very real risk of a nuclear disaster".
"We can't afford to lose any more time," the organisation's Director General Rafael Mariano Grossi said on Thursday.
"I'm determined to personally lead an IAEA mission to the plant in the next few days."
Ukraine energy minister adviser Lana Zerkal said the inspection "is planned for the next week, and now we are working on how they will get there".
But in an interview with Ukraine's Radio NV on Thursday evening she was sceptical the mission would go ahead, despite Moscow's formal agreement.
"They are artificially creating all the conditions so that the mission will not reach the site," she said.
Zelensky has said "the IAEA and other international organisations should react much quicker".
Energoatom said the plant outage was caused by a power line being twice disconnected by ash pit fires in an adjacent thermal power plant.
The three other power lines "were earlier damaged during terrorist attacks" by Russian forces, the operator said.
Energoatom did not disclose whether there were blackouts as a result of the power cut.
However the mayor of the city of Melitopol Ivan Fedorov said on Thursday "Russian occupiers cut off the electricity in almost all occupied settlements of Zaporizhzhia".
Kyiv suspects Moscow intends to divert power from the Zaporizhzhia plant to the Crimean Peninsula, annexed by Russian troops in 2014.
But on Thursday, Washington issued a direct warning against any such move.
"The electricity that it produces rightly belongs to Ukraine," State Department spokesman Vedant Patel told reporters.
"Any attempt to disconnect the plant from the Ukrainian power grid and redirect to occupied areas is unacceptable."
President Joe Biden, in a telephone conversation with Zelensky, also called for Russia to return full control of the plant and let in nuclear inspectors, the White House said.
Zelensky said he had spoken with Biden and thanked him for the United States' "unwavering" support.
Britain's defence ministry has warned that weekend satellite imagery shows an increased presence of Russian troops at the power plant.
Armoured personnel carriers were deployed within 60 metres (200 feet) of one reactor and "Russian troops were probably attempting to conceal the vehicles by parking them under overhead pipes and gantries".
"Russia is probably prepared to exploit any Ukrainian military activity near (the plant) for propaganda purposes," the ministry said.