By Vitalii Hnidyi and Anastasiia Malenko

KHARKIV, Ukraine (Reuters) -A Russian drone attack struck residential buildings in the Ukrainian city of Kharkiv and an energy facility in the surrounding region on Thursday, killing four people and severing power for 350,000 residents, officials said.

By evening, power supply limitations had been introduced in six Ukrainian regions due to the latest attacks on grid infrastructure and higher electricity consumption, grid operator Ukrenergo said. It described the limitations as "moderate".

Ukraine's second-largest city, which lies some 30 km (20 miles) from the Russian border, has been bombed heavily during the 25-month war and been one of the worst afflicted as Russia has renewed its missile and drone attacks on the energy system.

One of the killed rescuers was a 52-year-old firefighter whose son, also a firefighter, had been putting out a fire several buildings away, Interior Minister Ihor Klymenko said.

Realizing his father had been killed, the son, Volodymyr, knelt on the ground and wept as two emergency workers consoled him, video shared by Klymenko showed.

Under floodlights in the night, rescuers raced to free a resident trapped under rubble and ladders reached up from fire trucks to shattered apartments at the top of high-rise blocks.

President Volodymyr Zelenskiy called the attack "despicable and cynical" in a statement on X, repeating his call to Ukraine's allies to supply more air defences.

In his nightly video address, Zelenskiy said commanders had discussed defending Kharkiv "specifically in terms of air defence" and added it was a "task for all our diplomats" to persuade Ukraine's partners to provide more effective systems.

The president said top commander Oleksandr Syrskyi had reported "on all points where the Russian army had expected to have success". The military, he said, had stabilised the situation despite ammunition shortages and a "significant slowdown in supplies" from Ukraine's allies.

Regional governor Synehubov said Russia used at least 15 drones in the Kharkiv attacks, Synehubov said. The military shot down 11 Shahed drones out of 20 launched at the country overnight, the General Staff said.

Broadcaster Suspilne reported that one of the strikes caused serious damage to apartments on three floors of a 14-storey building. It said emergency crews had been unable to work for at least an hour for fear of further strikes.

Residential buildings, stores, a medical facility and cars were damaged in the attack, Kharkiv prosecutors office said.

"In Kharkiv and areas of the region, around 350,000 consumers have been disconnected," Ukrenergo said.

"Shahed attacks on energy facilities take place almost every day," Volodymyr Kudrytskyi, head of national grid operator Ukrenergo, told a news conference. "The intensity of the attacks has increased."

Russian forces also hit a solar power plant in Dnipropetrovsk region, causing a fire which was later put out, the Energy Ministry said. Some limits on energy consumption were put in place in the region in the morning, the officials said.

Russia targeted Ukraine's energy infrastructure with more than 150 missiles and over 240 Shahed drones between March 22-29 of this year, according to a statement from parliament's committee on energy, housing and utilities services.

These attacks damaged or destroyed at least eight power plants and several dozen substations, cutting off over 2 million citizens from electricity, heat and water supply, the statement said. Restoration of the facilities will likely take months.

Reuters was unable to verify the accounts. Russia denies deliberately targeting civilians in the war in which it is focusing on capturing eastern and southern parts of Ukraine.