As winter cold sets in across Ukraine, concerns are growing that Russia will soon resume large-scale attacks on the power grid, repeating a tactic it used last year to try to break the will of Ukrainians by plunging them into cold and darkness.

Those fears are compounded by what Ukrainian experts and current and former officials say is an energy system that is more fragile than it was a year ago. In interviews, they described power plants still hobbled by Russian attacks last winter, unfinished repairs to substations and shortages of critical equipment like transformers. And snow has already begun to fall.

The Ukrainian authorities declined to provide detailed data on the current state of the power grid, saying it was sensitive information in wartime. But experts say the situation has improved only marginally since a United Nations report published this summer estimated that Ukraine’s total generation capacity had fallen to half of its prewar level by late April.

“Not a lot has changed since then,” Victoria Voytsitska, a former lawmaker and senior member of the Ukrainian Parliament’s energy committee, said in an interview. “We’re in a much worse situation than last year.”

The situation looks particularly grim for thermal power plants, which are fired by coal or gas and are a key element in Ukraine’s energy mix to meet demand during peak consumption periods, the experts say.

Ukraine’s government says the plants will supply 4.5 gigawatts of power this winter, a third of the country’s prewar output, according to the United Nations. That is the same capacity that the organization estimated this summer, suggesting that there has been little progress in repair work since then.