Chris Graham and Rachel Muller Heyndyk
A "massive attack" by Russia against Ukraine's capital has killed four people and injured at least eight others, Kyiv Mayor Vitali Klitschko has said.
Drones hit several regions overnight, including Zaporizhzhia, where at least 16 people, including three children, were injured, authorities said.
Ukraine's foreign minister said hundreds of drones and missiles had been used in the latest attacks around Ukraine. Russia has not commented.
On Saturday, Ukraine's President Volodymyr Zelensky warned that Russia would not stop with his country - which is why it was testing European air defences with the recent incursions in several countries belonging to the Nato military alliance.
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in February 2022 and has been rebuffing international efforts - led by his US counterpart Donald Trump - to end the war.
"Putin will not wait to finish his war in Ukraine. He will open up some other direction. Nobody knows where. He wants that," Zelensky said.
He was speaking hours before the latest Russian barrage began hitting Ukrainian cities. Details are still emerging.
In neighbouring Poland, jets were scrambled early on Sunday as Russia hit western Ukraine, the Polish armed forces said.
The Polish military further described the actions - which have become routine since Polish and Nato aircraft shot down three Russian drones in Poland's airspace on 10 September - as preventative.
Moscow denied responsibility after Denmark said drones were flown over its airports. Denmark itself has said the incidents appeared to be the work of a "professional actor", without specifying who this may be.
Estonia has accused Russia of violating its airspace with warplanes.
After the incursions, Nato launched a mission to bolster its eastern flank.
Trump has gone as far as to say that Nato nations should shoot down Russian planes in their airspace.
He also shifted his position on the Russia-Ukraine war, saying for the first time last week that Ukraine could win all of its land back from Moscow.
In a speech delivered at the UN General Assembly on Saturday, Russia's Foreign Minister Sergei Lavrov said his country had no intention of attacking EU or Nato member states but warned of a "decisive response" to any "aggression" directed towards Moscow.