Ukraine's Zaporizhzhia nuclear plant occupied by Moscow's troops came back online on Friday afternoon, the state operator said, after Kyiv claimed it was cut from the national power grid by Russian shelling.

The plant -- Europe's largest nuclear facility -- was severed from Ukraine's power network for the first time in its history on Thursday due to "actions of the invaders", Energoatom said.

In an update, the operator said that as of 2:04 pm (1104 GMT) the plant "is connected to the grid and produces electricity for the needs of Ukraine" once again.

French President Emmanuel Macron warned "civil nuclear power must be fully protected".

"War in any case must not undermine the nuclear safety of the country, the region and all of us," he said during a visit to Algeria.

Separately on Friday, the EU presidency vowed to hold an emergency summit on the spiraling energy crisis caused by the war in Ukraine, which this week entered its seventh month.

The bloc has vowed to wean its 27 member states off Russian oil and gas in protest against the invasion.

However, anxiety over supply has sent prices soaring, and on Friday both Germany and France reported record electricity prices for 2023.

Prime Minister Petr Fiala said the Czech Republic, which holds the EU presidency, "will convene an urgent meeting of energy ministers to discuss specific emergency measures".

Energy anxiety

The Zaporizhzhia nuclear plant 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.

Energoatom said the outage was caused by ash pit fires at an adjacent thermal power plant, which damaged a line connecting the only two of the plant's six reactors in operation.

The three other power lines linking the complex to the national grid "were earlier damaged during terrorist attacks" by Russian forces, the operator said.

On Friday afternoon Energoatom said one reactor had been reconnected "and capacity is being added".

No time to lose

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 organization'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 skeptical 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 organizations should react much quicker".

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".

'Unacceptable'

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.

Armored personnel carriers were deployed within 60 meters (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.

© Agence France-Presse