KYIV, Ukraine — The Ukrainian energy agency responsible for the oversight and safe operation of the nation’s nuclear power plants said on Tuesday night that Russian hackers had launched their most ambitious effort yet on the company’s official website.
The attack appeared to fail and there was no indication that it threatened to disrupt the Ukrainian power grid or the company’s oversight of the nation’s 15 working nuclear reactors.
The company, Energoatom, said it had managed to keep the attack from being visible to users of the website. Even so, it was a reminder of the digital threat posed to the power infrastructure in Ukraine, where the shelling of the Zaporizhzhia Nuclear Power Plant has stirred global alarm.
Ukraine’s power grid has been knocked offline twice before, in 2015 and 2016, causing widespread blackouts, and Russian efforts to attack the Energoatom site were being closely watched in case they presaged a larger event.
The large-scale cyberattacks — like knocking out Ukraine’s power grid or frying cellular service — that Western intelligence officials worried would accompany the Russian invasion in February have failed to materialize. But Russia has used hacking campaigns to support its ground campaign, pairing malware with missiles in several attacks, including on TV stations and government agencies, according to a report released in April by Microsoft.
Ukrainian officials said this spring that they had thwarted a cyberattack on Ukraine’s power grid that could have knocked out power to two million people, raising fears that Moscow was continuing to persist in its digital efforts to advance its military campaign.
Daily shelling at the Zaporizhzhia complex — Europe’s largest nuclear power plant — has sparked international concerns about the possibility of a meltdown. The United Nations has offered to help send nuclear inspectors to the site, which is occupied by Russian forces but being operated by Ukrainian workers, if both sides agree.
It remains unclear who is to blame for the shelling. The Ukrainians have accused the Russians of directing strikes there to cut off energy supplies to other cities, while the Russians say Ukraine is responsible for the attacks.
On Tuesday, President Emmanuel Macron of France and President Volodymyr Zelensky of Ukraine spoke by telephone about safety threats at the Zaporizhzhia plant. According to a statement from Mr. Macron’s office, the French president stressed his concern about the impact of “ongoing clashes on the safety and security of Ukrainian nuclear facilities” and called for the withdrawal of Russian forces.
David E. Sanger contributed reporting.