Volodymyr Zelensky has accused Russia of breaching a unilateral ceasefire announced by Ukraine by launching a wave of drone attacks on several cities.
The Ukrainian president said Russia had carried out "active hostilities and terrorist shelling", targeting the front line as well as firing dozens of drones and missiles at civilian areas.
In the Sumy border region, two people were killed when a kindergarten was hit on Wednesday morning, local authorities said. No children were present at the time.
Earlier this week the two warring sides had announced rival unilateral ceasefires - with no agreement on their terms, length or monitoring.
Moscow was the first to demand a truce on 8 and 9 May to mark Victory Day on Saturday, when commemorations of the Soviet Union's victory over Nazi Germany are due to be held across the country.
Kyiv followed on by announcing its own open-ended ceasefire from midnight on Tuesday, and that Ukraine would act "symmetrically" from that point on.
On Wednesday Zelensky stated that Russia's choice was "an obvious spurning of a ceasefire and of saving lives".
Ukraine had "clearly stated that it would act in kind", he added, and would "decide on our further actions" based on evening reports from its military and intelligence.
Zelensky later said that Ukraine had observed that "additional rings of air defence" had been built up around Moscow, with systems redeployed from other parts of Russia.
This was proof the Kremlin was "more concerned about its parade in Moscow" than about the rest of the country, Zelensky said.
He noted that the fact that some parts of the Russia were left uncovered created "additional opportunities for our long-range sanctions" - an expression that most likely refers to Ukrainian strikes deep into Russia.
A slew of attacks on Tuesday left 27 people across Ukraine dead, including 12 victims in Zaporizhzhia alone.
Ukraine's foreign minister, Andriy Sybiha, said Russia's continued aggression showed "fake calls for a ceasefire on May 9th have nothing to do with diplomacy. Putin only cares about military parades, not human lives".
Various events are due to be held in Russia on Saturday, including a grand military parade on Red Square - although authorities have announced this year's event will be a pared-down version due to the "terrorist threat" from Kyiv.
Residents of Moscow and St Petersburg have also been warned their mobile internet access will be disrupted for security reasons.
The Kremlin, which launched its full-scale war on Ukraine in 2022, has long been concerned with sheltering the Russian population from the conflict.
But Ukraine has become ever more adept at targeting Russian cities, including Moscow, with long-range drones that cause limited damage but rattle public opinion.
On Tuesday Kyiv also launched strikes far into Russian territory, killing two people in Cheboksary, more than 1,500 km (930 miles) away from Ukraine.
Moscow has not addressed Ukraine's ceasefire demand. Instead, it has threatened a "massive missile strike" on central Kyiv if Ukraine violates Russia's unilateral 8-9 May truce.
The Russian defence ministry said it had downed 53 Ukrainian drones between 21:00 and 07:00 (18:00-04:00 GMT) overnight into Wednesday, but did not specificy whether any of them had been launched after Kyiv's unilateral ceasefire had come into effect.
