Ukraine’s nuclear state operator said Thursday that Russia has shelled and damaged power lines connecting Europe’s largest nuclear power plant to the Ukrainian grid, leaving the plant reliant on diesel generators again.

The generators have enough fuel to maintain the Zaporizhzhia Nuclear Power Plant in southeastern Ukraine for just 15 days, Energoatom said in a post on its Telegram channel.

“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.

As power lines and substations have been damaged in fighting, the plant has repeatedly been forced to rely on diesel generators until the power supply can be restored.

The International Atomic Energy Agency said Thursday that the switching to backup power from generators again further underlines “the extremely precarious nuclear safety and security situation at the facility and the urgent need to establish a protection zone around it.”

The development “again demonstrates the plant’s fragile and vulnerable situation,” IAEA Director-General Rafael Grossi said, adding that relying on generator power ”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.”

With its six reactors inoperative, the plant relies on outside electricity to cool its spent fuel. Russia and Ukraine have traded blame for months amid the war for shelling at and around the plant that the U.N.’s nuclear watchdog has warned could cause a radiation emergency.

The nuclear power plant lies within a part of the Zaporizhzhia region that has been occupied by Russian forces since the early days of the war, which began when Russia invaded Ukraine on Feb. 24.

Although Russian President Vladimir Putin signed a decree transferring the plant to Russian ownership, Ukrainian workers continue to run the plant. Energoatom has repeatedly called for the withdrawal of Russian forces from the plant and the creation of a demilitarized zone around it.

Energoatom said Russia shelled two power lines that were connecting the plant to the Ukrainian grid overnight, and accused it of being “an attempt to reconnect the nuclear plant to the Russian power system.” The operator said the Russian side would try to repair the power lines in order to connect the plant to the Russian grid and therefore supply power to occupied Crimea and the parts of the eastern Ukrainian region of Donbas also currently controlled by Russia.

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

Other Ukrainian cities were also hit, with Russia using drones, missiles and heavy artillery that left six civilians dead and 16 others wounded, according to the president’s office. Energy and water infrastructure facilities were hit in Zelenskyy’s native city of Kryvyi Rih, leaving several districts without electricity or water in the city that had a prewar population of 635,000 people, local Gov. Oleksandr Vilkul said.

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

Separately, seven ships carrying 290,000 tons of agricultural products set sail from Ukrainian seaports heading to Asia and Europe, a day after Russia agreed to rejoin a wartime agreement allowing Ukrainian grain and other commodities to be shipped to world markets.

In announcing Russia was rejoining the pact, Putin said Moscow had received assurances that Ukraine wouldn’t use the humanitarian corridors to attack Russian forces. He warned that Russia reserves the right to withdraw again if Kyiv breaks its word.

Kremlin spokesman Dmitry Peskov cautioned on Thursday that Russia’s decision to rejoin doesn’t mean that the deal will be extended after Nov. 19.

“Before making a decision to continue, of course, we will need to give an overall assessment of the effectiveness of the deal,” Peskov told journalists.

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

Ukrainian Foreign Minister Dmytro Kuleba told Spanish radio station Cadena SER that Ukraine has never used the grain corridor for military purposes, claiming that “people of goodwill” were behind the attack.

In Moscow, the Russian Foreign Ministry summoned U.K. Ambassador Deborah Bronnert on Thursday, saying she was called in connection with the alleged participation of British instructors in the Oct. 29 attack by drones on Black Sea fleet facilities in Sevastopol in Crimea. Bronnert made no comment upon leaving the ministry after a meeting that lasted about a half-hour.

Russian Foreign Minister Sergey Lavrov said Thursday he hadn’t seen progress regarding the export of Russian fertilizers and grain, despite the reimplementation of the Ukrainian part of the U.N.-sponsored grain deal.

Speaking to reporters at a joint news conference with his Jordanian counterpart Ayman Safadi, Lavrov also said Russia was pleased that the Ukrainian leadership had signed guarantees “that no attempts would be made to use humanitarian routes in the Black Sea for military purposes.”

Ukrainian Foreign Ministry spokesman Oleh Nikolenko denied that Kyiv had made such 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. “Our state did not take on any new obligations that would go beyond the limits of those already existing in the grain agreement.“

The ships that set sail Thursday included one carrying 29,000 tons of sunflower seeds bound for Oman, and one carrying 67,000 tons of corn heading to China, according to the Ministry of Infrastructure.

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