Russia has hit Ukraine with a wave of attacks, including Iranian-made "kamikaze" drones on the capital, Kyiv.
The strikes hit critical infrastructure in three regions, cutting off electricity in hundreds of villages across the country, according to Prime Minister Denys Shmygal.
At least seven people have died so far - four in Sumy and three in Kyiv.
A week ago, the capital was hit by Russian missiles at rush hour, part of nationwide attacks which left 19 dead.
Mr Shmygal said the new strikes had hit regions of Kyiv, Dnipro and Sumy.
Russia said it had hit "all designated targets", confirming that Ukraine's "military command facilities and energy system" had been targets.
Four people were killed when an energy-generating facility was hit in Sumy, according to Ukraine's Deputy Interior Minister, Yevhen Yenin.
"Russia is hunting for all energy-related facilities," Mr Yenin warned. "They want to cause chaos in the energy industry."
"It shows their desperation," said Andriy Yermak, head of Ukrainian President Volodymyr Zelensky's staff.
In the port city of Mykolaiv, sunflower oil tanks were set on fire by the suicide drones, said the city's mayor, Oleksandr Senkevich.
Mr Senkevich said three drones ignited the tanks at Mykolaiv late on Sunday evening, hours before the attack in Kyiv.
A Ukrainian air force official said since last night, 37 drones had been destroyed, which had all flown into the country from the south.
Monday morning's attacks in Kyiv were from drones - the low buzzing of these slow-moving weapons is becoming familiar across the country.
The city reverberated to the rattle of gunfire as anti-aircraft batteries frantically tried to shoot them down. Video on social media appeared to show one interception.
The explosions on Monday began at about 06:30 local time (03:30 GMT). The most recent was at about 08:10 local time.
Two were close to the city centre, with sirens and car alarms heard across the area.
"The enemy can attack our cities, but it won't be able to break us," said President Zelensky, describing the attacks as "terrorising the civilian population".
Kyiv's mayor, Vitaliy Klitschko, described the attacks as a "genocide of the Ukrainian people. The Russians need a Ukraine without Ukrainians."
Mr Klitschko said that the city authorities were expecting soon the arrival of air defence systems to protect the capital from drone attacks.
One Reuters journalist reported seeing fragments of a drone used in the attack that bore the words, "For Belgorod".
The Russian border city of Belgorod has been hit several times since Russia invaded Ukraine. Moscow blames Kyiv for the attacks.
Two gunmen - who had volunteered to fight in Ukraine - killed 11 people on Saturday at a Russian military training camp in the Belgorod region.
Writing on the Telegram social media site, Mr Klitschko said there were four strikes in Kyiv, although residents heard five or six explosions. He also told people to stay in air raid shelters.
But despite the warnings, the streets were far from deserted. Between the first and second set of strikes, plenty of people seemed to be going about their Monday morning business.
Mr Yermak described the kamikaze attacks as Russia's "death throes" and said Ukraine needed more air defence systems "as soon as possible".