KYIV, Ukraine — Russian shelling in the early hours of Saturday morning damaged the last line connecting the Zaporizhzhia Nuclear Power Plant to Ukrainian energy systems, according to Ukrainian energy officials, cutting the facility off from the power grid that is used to cool its reactors.
The plant has been disconnected from external power at least twice before, forcing it to rely on diesel generators to power safety equipment. The availability of fuel to power the generators has long been a concern.
Herman Galushchenko, Ukraine’s energy minister, said in a Facebook post on Saturday that there was only enough diesel fuel to operate the plant for about 10 days, adding that the professionalism of Ukrainian nuclear workers was now the only “safeguard against a possible nuclear accident.”
Energoatom, the company that operates the plant, in a statement shortly after midnight said that the power line was damaged “due to another shelling by Russian troops.” The statement said that it is “necessary to repair and restore the operation of the communication lines of the ZNPP with the power system,” but did not say how long that is expected to take.
Russia’s Ministry of Defense blamed Ukrainian forces for the shelling, saying in a statement on Telegram that Ukrainian rockets had hit near the industrial zone and knocked out the power supply.
The Kremlin’s recent announcement that it would nationalize the plant has drawn international denunciation, and on Saturday the European Union’s top foreign policy official, Josep Borrell Fontelles, in a statement condemned Russia’s seizure of the facility “in the strongest possible terms,” calling it “illegal.”
Zaporizhzhia, the largest nuclear plant in Europe, has been under the control of the Russian military since early March. But it has continued to be operated by its Ukrainian staff, even as a battle for control of the area has raged nearby, raising concerns about the potential for a nuclear catastrophe.
The news of the latest cutoff was later confirmed by Rafael Mariano Grossi, the director general of the United Nations’ nuclear watchdog, the International Atomic Energy Agency.
“The resumption of shelling, hitting the plant’s sole source of external power, is tremendously irresponsible,” Mr. Grossi said in a statement that did not assign blame for the shelling. He called the implementation of a security protection zone around the plant “an absolute and urgent imperative.”
Engineers had already begun work to repair the line, the agency said, noting that 16 of the plant’s diesel generators began operating automatically to provide the facility’s six reactors with power.
“After the situation stabilized, 10 of the generators were switched off, leaving six to provide the reactors with necessary electricity,” the agency said.
Mr. Grossi was in Kyiv on Thursday for talks with Ukrainian leaders and plans to visit Russia early next week to try to come up with a security arrangement for the site.
The nuclear agency has maintained a presence at Zaporizhzhia plant since early September, and a new team rotated in this week to provide independent observation and assessment of the plant.
Hours earlier, the I.A.E.A. said that a damaged power line had also temporarily forced one of the units onto its emergency diesel generators on Thursday.
External power is needed for cooling and other essential nuclear safety and security functions, the agency said. The plant had only one external power line available out of four lines available before the conflict, which was what was cut early on Saturday.
