Volodymyr Zelensky made the statement in a video address to the nation on Tuesday night.
He said that over the past 5 days, Russia had fired nearly 300 missiles and over 200 "Shahed" drones against Ukraine.
Zelensky added that Russia had planned the trajectories of its missiles and drones to "cause as much damage as possible."
It came after Russia on Tuesday morning launched an air attack on Ukraine's two biggest cities, Kyiv and Kharkiv, killing at least five people, according to Ukrainian officials.
On Friday, Russia launched the largest attack of the war, killing at least 39 Ukrainian civilians, the Reuters news agency reported.
More Russian air assaults on Ukraine followed on Sunday and on New Year's Day, according to news outlets.
Zelensky said on Tuesday night he had spoken to UK Prime Minister Rishi Sunak about "enhancing Ukraine's air defence" against Russia's air attacks.
Wednesday is day 679 of Russia’s war on Ukraine.
Russia invaded Ukraine on February 24, 2022, launching the largest military campaign in Europe since World War II.
(pm)
Source: Ukraine News, Reuters, president.gov.ua