KYIV, Ukraine — Ukrainian strikes disrupted power and heating to two major Russian cities near the Ukrainian border, local Russian officials reported Sunday.

The report comes as Russia and Ukraine have traded almost daily assaults on each other’s energy infrastructure and U.S.-led diplomatic efforts to stop the nearly four-year war have not advanced.

Elsewhere, Ukraine’s top diplomat accused Moscow of deliberately endangering nuclear safety, as he said Russia’s mass drone and missile attack on Friday struck substations that power two nuclear power plants.

And in Russia, the Kremlin spokesman said Moscow intended to honor its obligations under a global nuclear test ban, despite a recent order by President Vladimir Putin to study the possibility of resuming atomic tests.

A drone strike temporarily caused blackouts and cut heating to parts of Voronezh, regional Gov. Alexander Gusev said. He said several drones were electronically jammed during the night over the city, home to just over 1 million people, sparking a fire at a local utility facility that was quickly extinguished.

Russian and Ukrainian news channels on Telegram claimed the strike targeted a local thermal power plant.

A missile strike late on Saturday also caused “serious damage” to power and heating systems supplying the city of Belgorod, with some 20,000 households affected, local Gov. Vyacheslav Gladkov reported the following morning.

Russia’s defense ministry said Sunday that its forces destroyed or intercepted 44 Ukrainian drones during the night that flew over the Bryansk and Rostov regions in southwestern Russia. The statement made no mention of either the Voronezh or Belgorod provinces, nor did it specify how many drones Ukraine launched.

Local authorities in the Rostov region on Sunday reported on hourslong blackouts in the city of Taganrog, home to some 240,000 people, blaming them on an emergency shutdown of a power line. They did not specify the cause, though local media claimed a nearby transformer substation caught fire.

Months of Ukrainian long-range drone strikes on Russian refineries have aimed to deprive Moscow of the oil export revenue it needs to pursue the war. Meanwhile, Kyiv and its western allies say Russia is trying to cripple the Ukrainian power grid and deny civilians access to heat, light and running water for a fourth consecutive winter.