MOSCOW, Russia–President Vladimir Putin said on Monday that Moscow would intensify strikes on military targets in Ukraine, after an unprecedented Ukrainian attack over the weekend on the Russian city of Belgorod.

The Ukrainian attack on Saturday, which killed 25 people including five children, came after Moscow launched a large-scale attack on Ukrainian cities.

"We're going to intensify the strikes. No crime against civilians will rest unpunished, that's for certain," Putin said during a visit to a military hospital.

He said Russia would continue to hit what he called "military installations".

"We are doing that today and tomorrow we will continue doing it," Putin announced, speaking almost two years into Moscow's military offensive in Ukraine.

Meanwhile, Kyiv said Russia had targeted the country with a "record" number of drones on New Year's Day.

"What happened in Belgorod is a terrorist act," Putin told wounded Russian soldiers sitting near him in hospital pyjamas and sanitary masks.

"There is no other way to call it."

He accused Ukrainian forces of targeting "right in the city centre, where people were walking around, before New Year's Eve" and alleged they had "purposefully hit the civilian population".

Belgorod governor Vyacheslav Gladkov said Monday that the death toll had risen to 25, saying medics were unable to save a toddler who was seriously injured in the attack.

"Today in the local children's hospital a four-year-old girl, who was in a serious condition with severe injuries to the chest and internal organs, died," Gladkov said.

Her death brought the number of child victims of the attack to five.

"This is an irreparable loss for all of us," Gladkov said.

He said a total of 109 people were wounded, 45 of which are still in medical facilities.

Speaking about the situation on the battlefield, Putin said he believed the "strategic initiative" in the drawn-out conflict in Ukraine was on the Russian side.

"In any case, that is how I am being briefed," he said.

He also claimed Moscow wanted to end the conflict -- which has dragged on for almost two years -- "as quickly as possible" but "only on our terms", according to Russia's state-run TASS news agency.