Vladimir Putin made the declaration on Monday, the AFP news agency reported.
It came after an attack on the Russian city of Belgorod on Saturday killed 24 people and injured over 100, according to news outlets.
On Friday, Russia launched a large-scale air assault on Ukrainian cities, killing at least 39 people, Ukraine's authorities said.
On New Year's Day, Russia also hit Ukraine with a record number of drones, according to officials in Kyiv.
Putin on Monday vowed that Russia would "intensify the strikes" in response to the Belgorod strike, which he called a "terrorist attack."
The Kremlin leader said Russia would keep on hitting Ukraine's "military installations."
"We are doing that today, and tomorrow we will continue doing it," Putin stated, as quoted by the AFP news agency.
It came as Russia’s war on Ukraine passed the 677th day mark.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
(pm)
Source: AFP, DW, Kyiv Post