Belgorod, Russia - The Russian border region of Belgorod came under heavy fire from Ukraine on Saturday, resulting in at least 21 deaths and dozens of injuries, a day after Moscow launched a massive aerial assault on multiple Ukrainian cities.

Belgorod Governor Vyacheslav Gladkov said the Ukrainian army had fired on the center of the regional capital of the same name, and he told residents to hide in air raid shelters.

Gladkov said on Telegram that 21 people, including three children, were killed in the shelling and a further 110 people were injured, 30 of them seriously. The information could not initially be independently verified.

The governor added that mass events would be cancelled on the territory of the region's border municipalities, plus the Belgorod and Yakovlevsky urban district.

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Videos of Belgorod city on social media showed columns of black smoke and burning cars. In some recordings, explosions and the sound of people screaming could be heard.

Complaints about the response of authorities were also circulating, with some residents reporting that the air raid shelters they were supposed to take cover in were locked and inaccessible.

It was reportedly the heaviest shelling of Belgorod since Russian President Vladimir Putin ordered an all-out invasion of Ukraine on February 24, 2022.

Russia's Interfax news agency reported that Putin instructed Health Minister Mikhail Murashko to lead a team flying to Belgorod as part of the government's response to the crisis.

The Russian government also called for a special session of the UN Security Council to discuss the attack.

There was no immediate public comment from officials in Kyiv, but the newspaper Ukrainska Pravda wrote, citing an anonymous Ukrainian intelligence source, that the Ukrainian army had targeted Russian military objects in Belgorod.

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Civilians had come to harm due to "unprofessional actions of the Russian air defense as well as deliberate and planned provocations," the source said.

Belgorod borders Ukraine and has been repeatedly shelled – including at least once by Russia itself – since Putin ordered the all-out invasion of the country almost two years ago, although the damage and casualties are nowhere close to what Ukraine has experienced.

The attacks came after Russia unleashed its biggest aerial assault of the war on Ukraine on Friday, with cities across the country coming under fire from some 160 missiles and drones. At least 39 people were killed and 120 were injured, officials said.

Deaths were reported in Dnipro, Kharkiv, Zaporizhzhya, Odessa, Lviv, and Kyiv, where residents took shelter in the city's metro. Power was knocked out in several areas of the country.

"For every 'Shahed' drone, for every Russian missile, there will be a fair responsibility of the terrorist state," Ukrainian President Volodymyr Zelensky said in his evening address.

On Saturday evening, at least 26 people were injured as a result of Russian missile attacks on the eastern city of Kharkiv. A residential building in the city center and the Kharkiv Palace Hotel were hit, among other targets. Several people were taken to hospital.

The Ukrainian air force also said that the Russian army had again launched combat drones. Warnings were issued for the Kherson, Mykolaiv and Dnipropetrovsk regions.