Ukrainian forces on Monday swept deeper into territory seized from fleeing Russian troops, as joyful residents returned to former front line villages and Moscow grappled with the consequences of the collapse of its occupation force in Ukraine's northeast.

Ukraine's general staff said early on Monday that its soldiers had recaptured more than 20 towns and villages in just the past day, after Russia acknowledged it was abandoning Izium, its main stronghold in northeastern Ukraine.

"People are crying, people are joyful, of course. How could they not be joyful!" said retired English teacher Zoya, 76, north of Kharkiv in the village of Zolochiv 18 kilometers (11.15 miles) from the Russian frontier, weeping as she described the months she had spent sheltering in the cellar.

Nastya, 28, had fled the village in April but returned last week after news of Ukrainian advances: "I think everyone’s in a great (mood)! It’s all over now. At least we hope it’s all over," she said, queuing for groceries with two small children.

Further north, Ukrainian troops had moved into Udi, a hamlet in what had been no-man's-land closer to the frontier. Soldiers returning from there said it was still unsafe, littered with land mines, grenades and weapons left behind by fleeing Russian troops, with abandoned farm animals wandering about.

In what remained of Russian-held territory in the Kharkiv region, Vitaly Ganchev, the Russian-installed head of Moscow's occupation administration, acknowledged that Ukraine's troops had broken through to the frontier.

Ganchev, who has ordered the complete evacuation of civilians from Russian-held parts of the province, told Russia's state-owned Rossiya-24 television that about 5,000 civilians had escaped to Russia but the frontier was now shut.

"The situation is becoming more difficult by the hour," he said.

'Emergency defensive actions'

As thousands of Russian troops abandoned their positions, leaving behind huge stocks of ammunition and equipment, Russia fired missiles at power stations on Sunday causing blackouts in the Kharkiv, and adjacent Poltava and Sumy regions.

Ukraine denounced what it described as retaliation against civilian targets for its military advances. By Monday morning, Reuters journalists in Kharkiv said the power was back on, although the water was not yet working. The regional governor said power had been restored by 80%. Moscow, which denies deliberately striking civilian targets, did not comment.

Britain's ministry of defense said Russia had probably ordered its forces to withdraw from all of Kharkiv region west of the Oskil River, abandoning the main supply route that had sustained Russia's operations in the east.

Kyiv, which reached the Oskil when it seized the railway hub city of Kupiansk on Saturday, suggested Russia was already falling even further back: The Ukrainian general staff said Russian troops had abandoned Svatove in Luhansk province, around 20 kilometerseast of the Oskil. Reuters could not confirm this.

The British ministry said Moscow's forces were also struggling to bring reserves to the front line in the south, where Ukraine has launched a big advance in Kherson province aiming to isolate thousands of Russian soldiers on the west bank of the Dnipro River.

A spokesperson for Ukraine's southern command said Ukrainian forces had recaptured 500 square kilometers (193.05 square miles) of territory in the south. This could not immediately be independently confirmed.

"The majority of the (Russian) force in Ukraine is highly likely being forced to prioritize emergency defensive actions," the British update said. "The rapid Ukrainian successes have significant implications for Russia's overall operational design," it added.

Unraveling

Ukraine's swiftest advance since driving Russian forces away from the capital in March has turned the tide in the six-month war, unraveling in a matter of days swathes of the gains Moscow had achieved in months of costly fighting in the east.

Ukrainian chief commander Gen. Valeriy Zaluzhnyi said his troops had retaken more than 3,000 square kilometers this month, advancing to within 50 kilometers of the border with Russia.

Further Russian retreats, especially east of the Oskil, could soon put Ukrainian forces in position to attack territory that Russia and its local proxies have held since 2014.

Denis Pushilin, leader of the pro-Russian separatist administration in Donetsk province, acknowledged pressure from multiple directions.

"At the very least, we have stopped the enemy at Lyman," he said in a post on Telegram overnight, referring to a front line city east of Izium. "We'll have to see how that develops. But our boys have had clear successes."

He also described "successes" in fighting at Bakhmut, where Russia had long been concentrating its offensive, and Vuhledar further south.

Moscow has so far remained largely mute since its front line collapsed in the northeast last week, with President Vladimir Putin and his senior officials withholding any comment on the "special military operation" they have always said was "going to plan."

After days of making no reference at all to the retreat, Russia's ministry of defense acknowledged on Saturday that it had abandoned Izium and neighboring Balakliia, in what it called a pre-planned "regrouping" to fight in Donetsk.

Russian broadcasters, required by law to report only official accounts, have alluded to the setbacks but struggled to explain them, with commentators mainly demanding a redoubled war effort.

"We must win the war in Ukraine! We must liquidate the Nazi regime!" one commentator said on a panel show on NTV television.

"And how many years is that supposed to take?" replied another. "So my 10-year-old children will get a chance to fight?"