KYIV, Ukraine (AP) — Europe’s largest nuclear power plant was relying on emergency diesel generators to run its safety systems Thursday after external power from the Ukrainian electric grid was again cut off, Ukrainian and U.N. officials reported.

Fighting in Ukraine has repeatedly damaged power lines and electrical substations that the Zaporizhzhia Nuclear Power Plant requires to operate in-house safety systems, forcing operators to turn to backup generators to cool its six reactors until regular power is restored. All six reactors have been shut down. The generators have enough fuel to maintain the plant in southeastern Ukraine for just 15 days, state nuclear power company Energoatom said.

“The countdown has begun,” Energoatom said, noting it had limited possibilities to “maintain the ZNPP in a safe mode,” raising fears of a potential nuclear disaster.

The U.N.’s International Atomic Energy Agency confirmed the switch to backup diesel generators and said that underlines “the extremely precarious nuclear safety and security situation at the facility.”

The development “again demonstrates the plant’s fragile and vulnerable situation,” said Rafael Grossi, the director general of the U.N. nuclear watchdog, adding that relying on diesel generators ”is clearly not a sustainable way to operate a major nuclear facility.”

“Measures are needed to prevent a nuclear accident at the site. The establishment of a nuclear safety and security protection zone is urgently needed,” he said.

Russia and Ukraine have traded blame during the war for shelling at and around the plant. Energoatom said Thursday that Russian shelling knocked out the last two high voltage transmission lines feeding the Zaporizhzhia plant. Russia gave a different account, blaming Ukraine.

The Russian state-run news agency Tass quoted an official at Russia’s nuclear power operator, Rosenergoatom, as claiming that Ukraine had switched off the two power lines. The official, Renat Karchaa, confirmed that emergency backup diesel generators had to be switched on to run safety systems, but denied the problems had been caused by Russian shelling of power lines. He said the move deprived the city of Energodar, where plant’s workers live, of heating.

Russian forces have occupied the plant since the early days of the war. The plant is located in the Zaporizhzhia region, part of which has been occupied by Russian forces and illegally annexed, along with three other provinces, by Russian President Vladimir Putin last month.

Although Putin signed a decree transferring the nuclear plant to Russian ownership, Ukrainian workers continue to run the plant.

Energoatom said Russian officials are trying to connect the power station to Russia’s power grid so it could supply electricity to Crimea, which Russia annexed in 2014, and Ukraine’s Donbas region, annexed by Putin this fall.

Across the Dnieper River from the power plant, the city of Nikopol was also shelled again, damaging residential buildings, a gas station and several businesses, Ukrainian President Volodymyr Zelenskyy’s office said Thursday.

The U.N. nuclear agency is also tracking Russia’s unfounded claims that Ukraine is planning to set off radioactive “dirty bombs.” The IAEA said Thursday its inspections have found no evidence to support such claims after examining three locations in Ukraine.

Western nations have called Moscow’s repeated claim “transparently false.”

Russia used drones, missiles and heavy artillery to hit several Ukrainian cities, leaving six civilians dead and 16 wounded, according to the president’s office. Attacks in Zelenskyy’s native city of Kryvyi Rih left several districts without electricity or water.

Further east in the Donetsk region, battles continued for the towns of Bakhmut and Avdiivka, where authorities said the population was under constant shelling and living without electricity or heat. Over the past day, six cities and villages in the region came under attack from Russian heavy artillery, while in the northeast, three Russian missiles hit Ukraine’s second-largest city, Kharkiv, officials said.

On the humanitarian front, seven ships carrying 290,000 tons of agricultural products set sail from Ukrainian seaports for Asia and Europe, a day after Russia agreed to resume its participation in a wartime agreement allowing the export of Ukrainian grain. Putin said Moscow had received assurances that Ukraine wouldn’t use the humanitarian corridors to attack Russian forces.

Ukrainian Foreign Ministry spokesman Oleh Nikolenko denied that Kyiv had made any new commitments.

“Ukraine did not use and did not plan to use the grain corridor for military purposes. The Ukrainian side clearly adheres to the provisions of the grain agreement,” Nikolenko wrote on Facebook.

Kremlin spokesman Dmitry Peskov cautioned Thursday that Russia’s decision did not mean the deal would be extended after it expires Nov. 19.

Russia had suspended its participation in the grain deal last weekend, citing an alleged Ukrainian drone attack against its Black Sea fleet in Crimea. Ukraine didn’t claim responsibility for the attack, and Zelenskyy said Wednesday that Moscow’s return to the agreement showed “Russian blackmail did not lead to anything.”

In Moscow, the Russian Foreign Ministry summoned British Ambassador Deborah Bronnert on Thursday to protest what it alleges was the participation of British instructors in the Oct. 29 attack by drones on Black Sea fleet facilities in Sevastopol, Crimea. Bronnert made no comment after the meeting.

Under the grain export deal, Russia was supposed to be allowed to resume fertilizer and grain exports, but Russian Foreign Minister Sergey Lavrov said Thursday he hadn’t seen progress on that issue.

The ships that set sail Thursday from Ukraine included one carrying 29,000 tons of sunflower seeds for Oman and one carrying 67,000 tons of corn to China.

Since the deal was reached in July, 430 ships have exported 10 million tons of Ukrainian agricultural products to countries in Africa, Asia and Europe. Ukraine’s infrastructure ministry said export volumes in October “could have been 30-40% higher if Russia had not artificially blocked inspections.”

U.N. Secretary-General Antonio Guterres hailed the 10 million-ton milestone and appealed to all parties to renew the agreement.

“I’m not optimistic, I’m not pessimistic. I’m determined,” Guterres told reporters in New York, emphasizing it’s important to clear obstacles for Russian food and fertilizer exports, which include insurance and port guarantees for Russian ships that Western businesses have avoided.

The grain deal is one of the few areas where the warring parties are cooperating. Another is exchanges of prisoners and the bodies of war casualties. Both sides announced another prisoner exchange Thursday, this involving 107 military personnel on each side.

Elsewhere, a Ukrainian military official said Russia is using Belarusian territory to launch drone strikes. Oleksii Hromov, a representative of the Ukrainian military’s General Staff, said Iranian drones are flying into Ukraine from a military base in the Belarusian city of Luninets, 100 kilometers (62 miles) north of the Ukrainian border.

Belarusian President Alexander Lukashenko has supported Russia’s attack on Ukraine, prompting international criticism and sanctions against his government in Minsk.

Frank Jordans in Berlin and Edie Lederer in New York contributed.

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