President Volodymyr Zelensky says Ukraine has been hit by the biggest ever aerial attack from Russia - 728 drones and 13 cruise or ballistic missiles hit cities around the country in multiple waves.
Zelensky condemned the "telling attack", adding: "It comes precisely at a time when so many efforts have been made to achieve peace, to establish a ceasefire, and yet only Russia continues to rebuff them all."
The overnight strike came after President Donald Trump said the US would send more weapons to Kyiv - a reversal of last week's suspension which US media said Trump had not known about.
On Tuesday, the US leader expressed growing frustration at Russian President Vladimir Putin.
"We get a lot of bullshit thrown at us by Putin, if you want to know the truth," Trump told reporters. "He's very nice to us all the time, but it turns out to be meaningless."
Kremlin spokesman Dmitry Peskov said Moscow was "pretty calm about this. Trump's way of talking is generally quite harsh, the phrases he uses."
The two leaders have been in regular contact but this has so far failed to translate into tangible steps towards a ceasefire in Ukraine - something Trump once said he would be able to achieve in a day.
Last week, following a phone call with the Russian president, Trump said he was "very unhappy".
"He wants to go all the way, just keep killing people, it's no good," Trump said of Putin.
The criticism came even as the Trump administration announced a suspension of military aid to Ukraine, reportedly authorised by US Defence Secretary Pete Hegseth and Elbridge Colby, the under-secretary of defence for policy.
Asked by reporters on Tuesday who had taken the decision, Trump - sitting right next to Hegseth - replied: "I don't know. Why don't you tell me?"
The reversal of the decision may now mean that 10 Patriot missiles may be sent to Ukraine, according to US outlet Axios.
Kyiv relies on the interceptors to try to counter Russia's missile and drone attacks, which continue to grow in intensity and frequency.
Although the east of the country and Kyiv come under fire on a regular basis, no corner of Ukraine has been spared by Russian strikes.
The city of Lutsk - which lies 90km (56 miles) from the Polish border and is a transit hub for military and humanitarian aid - suffered the brunt of Tuesday's overnight attack.
Explosions were also reported in the western cities of Lviv and Rivne.
Two rounds of ceasefire talks between Russia and Ukraine took place earlier this year but no other meetings have so far been scheduled - and neither Moscow nor Kyiv appear optimistic that diplomacy will solve the conflict, which was sparked by Russia's full-scale invasion of Ukraine in February 2022.
Meanwhile, Russia's summer offensive in eastern Ukraine grinds on.
"We are moving forward," said Peskov on Wednesday. "Each new day the Ukrainians have to accept the new realities."