Yet by Monday, Moscow had announced air, rocket and artillery attacks on reclaimed areas in the Kharkiv region, a day after Kyiv said Russian strikes on electricity infrastructure had caused power failures.
The retaliatory fire came as Ukraine said forces had retaken more than 20 additional settlements, claiming "Russian troops are hastily abandoning their positions and fleeing".
Kyiv had already announced the recapture of Izyum in the country's east, while President Volodymyr Zelensky said Monday that Ukraine's forces retook a total of 6,000 square kilometres (2,320 square miles) from Russian control in September.
US Secretary of State Antony Blinken on Monday said Ukrainian forces had made "significant progress", due to their resilience as well as US support.
"It's too early to tell exactly where this is going. The Russians maintain very significant forces in Ukraine as well as equipment and arms and munitions. They continue to use it indiscriminately against not just the Ukrainian armed forces but civilians and civilian infrastructure as we've seen," Blinken said.
A US think tank, the Institute for the Study of War, tweeted: "Ukraine has turned the tide in its favour, but the current counter-offensive will not end the war."
Defence minister Oleksii Reznikov told French daily Le Monde in a Monday interview that the war has entered a new phase with the help of Western weapons.
Moscow conceded having lost territory -- which experts saw as a serious blow to its war ambitions -- but Kremlin spokesman Dmitry Peskov said there was no prospect of negotiations.
"The special military operation continues and will continue until the objectives that were originally set are achieved," he added, using Russia's terminology for the internationally condemned war.
'Weapons, weapons, weapons'
The Russian strikes hit 15 locations on Sunday, from Kramatorsk in the east to Mykolaiv in the south and Dnipro in between, Ukraine's military said.
Ukraine had already lost all power from the Russian-controlled Zaporizhzhia nuclear plant, threatened by shelling since February's invasion.
The country's nuclear energy agency said the final reactor at the plant -- Europe's largest nuclear power station -- had been shut off as a safety measure.
Kyiv and Moscow have shown "signs that they are interested" in creating a security zone around the plant, the UN atomic watchdog said Monday.
"What we need here really is Ukraine and Russia to agree on a very simple principle of not attacking or not shelling the plant," IAEA director general Rafael Grossi told reporters.
But later Monday, Ukraine foreign ministry spokesman Oleg Nikolenko said the only way to "ensure the nuclear safety and security of the Zaporizhzhia power plant is its de-occupation, demilitarisation and return to Ukrainian control".
"All IAEA efforts must be focused on achieving this goal," he wrote on Twitter.
The speed of Ukraine's fightback has seemingly caught Russia's military off guard, bringing swathes of territory Moscow had controlled for months back into Kyiv's fold.
Images posted by the Ukrainian military showed crates of munitions and military hardware scattered across territory abandoned by Russian forces.
Around the town of Balakliya, AFP journalists saw evidence of fierce battles, with buildings destroyed or damaged and streets mostly deserted.
Ukrainian authorities also claimed to have found four bodies of civilians with "signs of torture" in the recaptured village of Zaliznychne.
Residents reported that Russian troops had killed villagers, the regional prosecutor's office said.
Ukraine's foreign minister used the momentum of the country's fightback to appeal to Western allies for more stockpiles of sophisticated weapons.
"Weapons, weapons, weapons have been on our agenda since spring. I am grateful to partners who have answered our call: Ukraine's battlefield successes are our shared ones," Dmytro Kuleba said.