Russia fired almost 400 long-range drones at Ukraine overnight, Ukraine's air force said, in its biggest attack in weeks.
The onslaught continued into Tuesday morning as dozens of drones targeted the capital Kyiv during daylight.
Russia also launched 23 cruise missiles and seven ballistic missiles at Ukraine during the night, hitting at least 10 locations across the country, according to the air force.
Ukrainian civilians have endured relentless barrages since Russia launched a full-scale invasion of its neighbour more than four years ago.
US-brokered talks between Moscow and Kyiv in the past year have brought no respite, with Russia rejecting Ukraine's offer of a ceasefire, and in recent weeks the Iran war has diverted international attention from Ukraine's plight.
On the roughly 1250km front line snaking along eastern and southern parts of Ukraine, the short-handed defenders have been bracing for a new offensive by Russia's bigger army as the weather improves.
The commander-in-chief of Ukraine's armed forces, General Oleksandr Syrskyi, said Russian troops in recent days have made simultaneous attempts to break through defensive lines in several strategic areas.
"Fierce fighting unfolded along the entire line of contact," Syrskyi said Monday on the Telegram messaging app, with Russia launching 619 attacks in four days.
"The occupiers are attempting to bring up new units and are preparing to continue attacks," Syrskyi said, adding that Ukraine had deployed reinforcements to counter the assaults.
The Institute for the Study of War, a Washington-based think tank, said Syrskyi's report backed up its assessment that Russia's spring-summer offensive was now under way.
Russia had escalated its strikes since March 17 and had moved heavy equipment and more troops to the front line, the institute said.
Each year, as the weather improves, Russia has moved its grinding war of attrition up a gear.
However, it has been unable to capture cities and has made only incremental gains across rural areas.
Russia occupies about 20 per cent of Ukraine.
That includes the Crimean Peninsula, which Russia seized in 2014.
Ukraine has developed advanced drone technology to make up for its shortage of infantry.
Amid the Middle East conflict, Kyiv is offering Ukraine's battle-tested drone defences to US and Gulf partners, hoping to trade that know-how for scarce Patriot air defence missiles it needs to fend off Russia's barrages.
Ukraine has also used its domestically produced long-range drones to hit areas of Russia that support Moscow's war effort.
Russian air defences intercepted 55 Ukrainian drones overnight over Russian regions, the annexed Crimea and the Black Sea, the Russian defence ministry reported.
