Russia pounded Ukraine with missile and drone attacks overnight on Saturday and into Sunday morning, focusing on the major western city of Lviv.
Ukraine's neighbour Poland scrambled fighter jets in order to ensure the safety of Polish airspace, the Polish military confirmed. Allied Nato aircraft were also deployed.
Lviv's regional head Maksym Kozytskyi said two people were killed in strikes in the region, and two more injured.
Elsewhere, Russia again targeted Ukraine's power plants - and one was struck in an overnight attack on Zaporizhzhia, where the mayor said one person died and more than 73,000 people were without electricity.
The Russian assaults came days after a US official said the US would support Ukraine launching deep strikes inside Russian territory.
Poland's operational command stated, Polish and allied aircraft are operating in our airspace, while ground-based air defence and radar reconnaissance systems have been brought to the highest state of readiness.
At 05:10 (02:10 GMT), all of Ukraine was under air raid alerts following Ukrainian Air Force warnings of Russian missile and drone attacks.
Russia has stepped up its attacks on Ukraine's energy infrastructure, with Lviv's mayor noting that part of the city - 70 km from the Polish border - had no power. The city's air defence systems were engaged heavily in repelling initial drone and then missile attacks.
As of 07:30 (04:30 GMT), parts of the city were left powerless and public transport remained halted, prompting officials to stress it was dangerous to go out into the streets.
The aftermath of the attack in Zaporizhzhia left over 73,000 residents without electricity, with reports of casualties including a 16-year-old girl receiving medical assistance.
In Russia, air defence units reportedly destroyed 32 Ukrainian drones overnight, according to state-owned RIA news agency.
Meanwhile, Lithuania briefly closed its airspace after foreign objects were spotted, causing temporary flight suspensions at its largest airport.
