Ihor Klymenko said 26 people were still missing after the overnight strikes, including three children. He said nearly 100 people had been hurt.
Russia fired 476 drones and 48 missiles at Ukraine, striking energy and transport infrastructure and forcing emergency power cuts in a number of regions in frigid temperatures.
The upper floors of a residential building in Ternopil were torn away in the attack. Smoke poured upwards as fire crews tried to douse flames, while devastated residents huddled outside waiting for news of loved ones.
Klymenko, writing on Telegram, said emergency crews were working through the night, combing the site of the shattered apartment building.
"A lot of work lies ahead. The main thing is to find those who could still be under the rubble," Klymenko wrote.
"In the building where two entrances were completely burned out, not a single apartment was left intact. The flames flared up instantly and engulfed the building in a wave. People were terrified and tried to jump out of windows."
Officials said three children were among the dead.
Poland, a NATO member state bordering western Ukraine, temporarily closed Rzeszow and Lublin airports in the southeast of the country and scrambled Polish and allied aircraft as a precaution to safeguard its airspace.
Russia launched the attack as Ukrainian President Volodymyr Zelenskiy travelled for talks in Turkey intended to help revive peace negotiations with Russia, after his short tour to European capitals.
A senior Ukrainian official told Reuters that Ukraine had received "signals" about a set of US proposals to end the war that the United States has discussed with Russia.
Ukraine has had no role in preparing the proposals, the source said.
No face-to-face talks have taken place between Ukraine and Russia since a meeting in Istanbul in July and Russian forces have pressed on with the Kremlin's nearly four-year-old war in Ukraine, killing 25 people in strikes overnight.
Efforts to revive peace negotiations appear to be gaining momentum although Russia has shown no sign of changing its terms for ending the war.
Zelenskiy met President Tayyip Erdogan after visits to Greece, France and Spain that went ahead despite a political crisis in Ukraine over a corruption scandal in which parliament dismissed the energy and justice ministers.
Zelenskiy said after meeting Erdogan on Wednesday that Ukraine hopes to revive prisoner of war exchanges with Russia by the end of the year.
"Of course, we talked substantively about the situation in diplomacy. And now many processes have become more active, and we are trying to ensure that all activity is aimed specifically at peace," he said.
Zelenskiy urged allies to increase pressure on Russia to end its nearly four-year-old war in Ukraine, including by providing Kyiv with more air-defence missiles.
"Every brazen attack against ordinary life shows that the pressure on Russia is insufficient. Effective sanctions and assistance to Ukraine can change this," he said on X.
Russian forces control about 19 per cent of Ukrainian territory and are grinding forwards while carrying out frequent attacks on Ukrainian energy infrastructure amid winter.
Turkey, a NATO military alliance member that has remained close to both sides, hosted an initial round of peace talks in the early weeks of the war in 2022 - the only such talks until this year when US President Donald Trump launched a new bid to end the fighting.
