Russia is building power lines in occupied southeastern Ukraine to link to its own grid a major nuclear plant it has captured, according to a new Greenpeace report. It is the clearest evidence yet of Moscow’s intent to restart and exploit the offline facility, despite the risks and calls to address the plant’s status in peace talks.

The facility, the Zaporizhzhia nuclear power plant, was seized by Russia early in the war in a move widely condemned by the international community. Its proximity to frontline fighting has raised fears of a potential nuclear disaster, and experts have warned against any attempt to restart the plant under current conditions.

The Greenpeace report, which was shared with The New York Times, includes satellite images showing that, since early February, Russia has been building more than 50 miles of electricity lines and pylons between the occupied Ukrainian cities of Mariupol and Berdyansk, along the coast of the Azov Sea. The satellite images were verified by The Times.

Based on the location and direction of the work, Greenpeace said the project aimed to link the new power lines to a large substation near Mariupol that was connected to the Zaporizhzhia nuclear plant, about 140 miles farther west.

“Putin’s plan for restarting the Zaporizhzhia nuclear plant depends on securing new electricity transmission lines — this is the first physical evidence of those plans,” Shaun Burnie, a nuclear specialist at Greenpeace Ukraine, said in an interview.

Moscow’s exact plans remain unclear. There are questions about whether it seeks to run the plant in a postwar Ukraine or to do so while fighting is still underway. In either case, experts note, Russia would need to build several more lines to connect the Zaporizhzhia plant to its own grid, a process that would take time.

Olga Kosharna, an independent Ukrainian nuclear expert, said linking the plant to its own grid has been a longtime goal for Moscow, as expressed in official statements throughout the war.

It would be the first time a warring nation seized another country’s nuclear facility and then used it for its own energy needs. And it would go against recent efforts by the Trump administration to discuss the fate of the plant as part of possible peace talks.

Mr. Trump has expressed interest in the United States taking control of Ukrainian nuclear plants, citing safety concerns and their economic potential. Last month, the White House presented a peace plan to Kyiv and its allies calling for Russia to return the plant to Ukraine, but under U.S. management. Under that plan, the facility would supply electricity to Ukraine and Russia.

Russia has flatly rejected the idea, with Foreign Minister Sergey V. Lavrov telling CBS News that the Zaporizhzhia plant was being run by the Russian nuclear giant Rosatom and that he did not think “any change is conceivable.” Rosatom and Russia’s energy ministry did not immediately respond to a request for comment about the building of new power lines.

The recent power line construction suggests that Russia is not aiming just to hold onto the plant, but also wants to harness it to power its own grid.

Built during the Soviet era, the Zaporizhzhia facility is Europe’s largest nuclear power complex. Its six reactors can generate up to six gigawatts of electricity — enough to power all of Portugal — and they supplied nearly a quarter of Ukraine’s electricity before the war began in 2022.

The plant sits in the Russian-controlled part of Ukraine’s southern Zaporizhzhia region, dangerously close to the front lines, making safe operation impossible. All six reactors were gradually shut down after Russia seized the site, with the last one closing in 2023.

Russia has signaled its intent to power it back up, at one point citing 2024 as the target year to bring it online.

“Everyone is living with the dream of restarting the plant,” Rosatom’s director general, Alexei Likhachev, said last week, according to the Russian news agency Interfax. A plan had been developed to return the Zaporizhzhia facility to full capacity, he said.

One of the main challenges to restarting the plant, Mr. Likhachev noted, was the need to “replace the power grid.”

Indeed, of the four 750-kilovolt lines that once connected the plant to Ukraine’s grid, two pass through Ukrainian-held territory. The other two, on Russian-occupied land, have been damaged by the fighting and only one may have been repaired, according to Ms. Kosharna, the nuclear expert.

That leaves Russia without enough lines to fully tap the plant’s generation capacity. “They need to build more of them,” Mr. Burnie, of Greenpeace, said.

Mr. Burnie said one possible goal for Moscow was to eventually connect the Zaporizhzhia plant to the power grid in Russia’s Rostov region, which borders occupied areas of eastern Ukraine.

Satellite images obtained by Greenpeace show new power lines being built across fields near Mariupol, as well as the distinctive triangular shape of transmission towers. The latest imagery, from May 11 to May 22, shows these lines expanding east of the village of Shevchenko, about seven miles from a substation linked to the Zaporizhzhia nuclear plant.

Restarting the Zaporizhzhia nuclear plant would carry significant risks, energy experts say.

After more than three years of war, critical equipment remains unreplaced and many experienced Ukrainian staff members have fled. The 2023 destruction of a nearby dam on the Dnipro River, probably by Russia, also deprived the plant of the main water source needed to cool its reactors and its spent fuel rods.

Herman Galushchenko, Ukraine’s energy minister, said in a statement that “any attempts by Russian representatives to restart power units could lead to unpredictable consequences.”