Moscow and Kyiv said civilians had been killed in the latest exchange of air strikes Wednesday, and Russian President Vladimir Putin vowed to restore security after a sharp increase in attacks on Russia's border regions.

The Russian regions of Belgorod and Kursk just across the border from Ukraine have come under intense attack over the past 10 days. Kyiv has launched a barrage of drone and missile strikes and pro-Ukrainian paramilitaries have attempted armed raids into Russian territory.

Fresh attacks on Belgorod killed three civilians Wednesday, the Russian governor said.

In Ukraine, a Russian missile strike on the Ukrainian city of Kharkiv killed at least five, the regional prosecutor's office said.

Another seven were wounded, Oleg Synegubov, the governor of Kharkhiv, said earlier, and a senior police official warned the toll could rise.

"The city was hit by an X-59 missile. Rescue and firefighting operations are underway. There are still people under the rubble," the Kharkiv regional prosecutor's office said.

Kharkiv, Ukraine's second-largest city, lies close to the Russian border, and has seen increased attacks in recent months as Moscow's invasion moves into its third year.

- 'Massive strikes' -
Sergiy Bolvinov, head of the investigative department of Kharkiv's police, said of the deadly strikes on Kharkiv that an eight-storey building and a factory has been hit at around 1:00 pm local time (1100 GMT).

"Unfortunately, the number of casualties may increase," he added.

Ukraine's emergency services published images of at least five fire engines working at the scene, with a fire burning on the top floor of the building.

Directly across the border in Russia's Belgorod region, the governor said multiple attacks on Wednesday had killed three people.

"Since early morning, the Graivoron district has come under massive strikes, including with the use of multiple rocket launcher systems," Governor Vyacheslav Gladkov said in a post on Telegram.

Two people were killed there, and another man was killed in the regional capital, also called Belgorod, when shrapnel from a shelling attack hit his car, Gladkov said.

Some schools would shift to remote learning, a day after he ordered 9,000 children to be evacuated from areas closest to the Ukrainian border, he added.

The region has been hit with frequent shelling and drone strikes since Russia invaded Ukraine in February 2022, but attacks stepped up earlier this month ahead of Russia's presidential elections.

- 'Victory' -
In Moscow, Putin vowed he would restore order to Russia's border regions, as the fallout from his invasion continues to spill into Russian territory.

He also said his win in a weekend presidential vote was a "prologue" to victory in Ukraine.

He spoke inside the Kremlin's gilded Andreyev Hall, addressing election officials after winning another six-year term in office in a vote with no genuine opposition.

"We will do everything to support people who lost their businesses and homes, we will do everything possible," Putin said.

"But the first thing, is of course to ensure security. There are different ways, they are not easy, but we will do them," he added, without elaborating.

"Victory in the elections is just a prologue to those victories that Russia so badly needs and that will definitely come," Putin said, two years into the Ukraine offensive.

Russian forces have been advancing on the battlefield, securing their first territorial gains in almost a year, as Kyiv faces shortages of manpower and ammunition.

Russia's army claimed this week to have made further advances around Avdiivka, the front line town in the eastern Donetsk region that Moscow captured last month.

In the south of the country, Russian shelling killed two people outside the city of Kherson Wednesday, the local Ukrainian governor said.