Already, more than a third of Ukraine’s hard-to-replace transmission hubs have been damaged or destroyed, officials said.

Russia’s shift in tactics is alarming Ukrainian and Western officials as temperatures start to drop in Ukraine. They warn that the attacks could inflict suffering on civilians, create a new wave of refugees and further erode Ukraine’s war-shattered economy. Many Ukrainian cities are heated from centralized plants that require both electricity and gas to function, meaning the attacks could be particularly devastating.

Western officials have condemned the attacks on infrastructure as a war crime, saying they are intended to sow terror in the civilian population. The campaign has been relentless and highly strategic — unlike the Russian military’s ground tactics, which often seem ill-conceived, Ukrainian officials said.

“All the drones they’re using, missiles, everything is targeting energy infrastructure,” Ukrainian Energy Minister German Galushchenko said in an interview. “They have some kind of road map for the militaries, where to shell. If they missed one day, then the next day they shell it again and again.”

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The attacks are also proving enormously difficult to defend against, and officials said there was little they could do to harden the system against the strikes, which Russia has conducted with barrages of long-range missiles and attack drones.

“The goal of this is to create the most possible obstacles to reconnect quickly,” Galushchenko said. “Every day, shelling to infrastructure makes us closer to bigger problems.”

Another goal is to broadly hobble Ukraine’s ability to support its troops on the front lines.

Ukraine’s backers in Europe and Asia have promised to provide more powerful air defense systems and to rush equipment and other assistance to help rebuild critical infrastructure. But many of the air defense systems are complicated to use, require extensive training and have been slow to arrive.

Previously, when power plants or transmission lines came under attack, Ukrainian energy officials were able to reroute electricity around the problem, using their country’s thick web of Soviet and post-Soviet energy infrastructure to bypass problems. But that resilience is eroding quickly, officials said.

And repairs to the damaged infrastructure are pointless so long as Russia can attack the same targets again and again. Most of the substations and transformers need to sit aboveground and many need to be clear of obstructions around them, making them easy targets.

“The rules of the game are unfair,” said Volodymyr Kudrytskyi, the chief executive of Ukrenergo, the country’s main grid operator. “It is much quicker and easier to launch a missile and destroy the equipment or the object than to renovate it.”

Replacing specialized transformers and other substation infrastructure is especially difficult because they often must be custom-built, a process that can take months, experts said.

Kudrytskyi and others said they saw the spectral presence of their Russian energy counterparts in the decisions behind what is being hit, as though people just like them were planning the strategy. Russia and Ukraine’s grids are technically similar, since they were part of the same country until 1991, and Soviet-era infrastructure maps can still provide a road map to destruction.

“They are obviously targeting those substations and power plants which are most crucial for some regions, particular regions or for the power system in general as a whole,” Kudrytskyi said. They know “where to strike to inflict as much damage as possible. Because their target is terror. Their intention is to disconnect as many people from the grid as possible to create this panic.”

For now, Kudrytskyi said, 90 percent of Ukrainians have had their power restored within a day of an attack. “The problem,” he said, “is that the safety buffer of the system is getting lower. At the current rate of destruction, there is no such stock that could be sufficient to last for months or years.”

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Authorities have begun asking residents to stop using power-hungry appliances, and they have imposed planned blackouts of several hours at a time in Kyiv and cities around the country.

Many local governments have switched away from electric trolley-buses to diesel-powered ones, one of several measures they are taking to conserve electricity. The scheduled blackouts help ease the burden on the grid and give energy companies precious hours to scramble repair teams and reroute electricity flows across the undamaged parts of their transmission network.

“My personal assessment is that they can hardly create a total blackout in the country,” said Olena Pavlenko, the president of DiXi Group, a Kyiv-based energy consultancy. “There will still be a possibility to have electricity supply in all regions. But they will create a situation where we have longer interruptions of electricity supplies in the cities.”

The attacks have started to create a new calculus among Ukrainians.

For those in the east and center of the country, many of whom had only recently returned to their homes after spending months abroad or in the country’s west, it raised the possibility that they might need to flee again. Even for those who intend to stay, conversations have begun over what needs to be done to prepare for a winter potentially without heat and electricity for extended periods.

“When you have to stay without electricity, you have this feeling that you are in constant danger,” said Pavlenko, who added that her own apartment in Kyiv had been without power for four hours that afternoon. “You are not able to live as you lived before. It’s terrorizing in all regions.”

One recent news report advised residential buildings to place emergency packages in elevators, in case inhabitants found themselves stuck between floors during a power outage. In one apartment building, the contents included a flashlight, water, cookies, as well as two adult diapers and a light sedative.

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At the bottom of the list of contents on the package, a request was written: “Please, do not use the contents if you don’t need to, and replace what you use.”

Ukraine’s power generation capacity plunged in the early weeks of the war after Russia captured its Zaporizhzhia Nuclear Power Plant, the largest atomic power stations in Europe. But with much of the country’s industry idled by the conflict, power demands are also far lower than during peacetime.

Ukraine is still able to generate enough electricity for its needs — and until just two weeks ago, was actually exporting its surplus to European neighbors. But its ability to move electricity from power plants, many of which are in Ukraine’s north and west, to the places where it is needed, near the front lines in the south and east, is rapidly diminishing.

“The main target of the Russians’ attack is to create a situation the Ukrainian system can’t work jointly,” said Oleksandr Kharchenko, the managing director of the Kyiv-based think tank Energy Industry Research Center. “They want to split it into several parts. We can clearly see this plan.”

Another objective — after Russia has run into battlefield challenges on the front lines and is retreating from the southern city of Kherson and other areas — is to undermine the Ukrainian military from the rear.

“This is a completely different way how Russia is now targeting infrastructure,” said Artur Lorkowski, the director of the Vienna-based Energy Community Secretariat, an international organization affiliated with the European Union that has been coordinating efforts to direct spare parts and infrastructure assistance to Kyiv. “This is something that makes me scared about the future.”

Lorkowski said targeting the energy network could lead to civilian suffering that outstrips the already grievous toll of the war, which on Monday entered its ninth month.

“I would like to be wrong, but if the intensity of the shelling is kept by the Russians, you could expect a really, really tough winter,” Lorkowski said in a phone interview from the Polish-Ukrainian border, where he was returning after a visit to Kyiv focused on aid efforts. “They’re trying to push the people to a crisis situation through limited or no access to electricity and heat during wintertime.”

The attacks on energy infrastructure have led to calls for allies to step in to help, both with air defenses and with spare parts for the power system.

The Biden administration said it was trying. “We are working with the Ukrainians and regional and allied partners to see what can be done to shore up some alternative sources of energy for them as winter approach,” National Security Council spokesman John Kirby said. He added that the United States was working hard to make sure Ukrainians can “improve their air defense capabilities.”

Poland recently presented the European Commission with a list of Ukraine’s most urgent infrastructure needs.

The list, drawn up with Kyiv, outlines the need for items such as mobile cranes, vehicles for transporting reinforced concrete poles, miles of power cable and more than a dozen types of transformers, as well as submersible pumps, surge limiters and chain saws, among other things.

A Polish diplomat, speaking on the condition of anonymity to discuss ongoing talks, said diplomats from E.U. member countries had been briefed on the letter. “More and more member states understand the situation and I think they want to help,” the diplomat said.

Some of the needed material could be sourced from the European Union, the diplomat said, while other items may need to be ordered from elsewhere, potentially with financial support from E.U. countries. Even before the latest round of Russian attacks, E.U. nations were donating generators, repair kits and transformers.

Kudrytskyi, the Ukrenergo chief executive, said he felt he was in a race to make repairs faster than Russian shelling could destroy his work. “It’s a very dangerous situation,” he said, “and we do not know their abilities for destruction.”

Stern reported from Kyiv, and Rauhaula reported from Brussels. Beatriz Ríos in Brussels contributed to this report.