The Ukrainians this month began a counter-offensive in Eastern Ukraine that has forced Russian troops to retreat from large tracts of land they had occupied for months.

Ukraine's military chief General Valerii Zaluzhnyy said Ukrainian forces had recaptured about 3,000 square kilometers (1,160 square miles) since the counteroffensive began, adding that soldiers only 50 kilometers (about 30 miles) from the Russian border.

One battalion shared a video of Ukrainian forces in front of a municipal building in Hoptivka, a village just over a mile from the border and about 19 kilometers (12 miles) north of Kharkiv.

Kharkiv Governor Oleh Syniehubov said Ukrainian troops have reclaimed control of more than 40 settlements in the region.

The recaptured territories include Izyum, a major base for Russian forces in the Kharkiv region. Ukrainian forces also liberated Balakliya town after seven months of Russian occupation. Videos surfaced on the internet showing Ukrainian people greeting Ukrainian soldiers who had driven the Russians out.

Ukrainian Foreign Ministry spokesman Oleh Nikolenko also suggested troops had retaken Kupiansk, a town along the main supply route to Izyum.

Following these military reversals, Russia on Sunday bombed power stations across Russia. The bombardment ignited a massive fire at a power station on Kharkiv's western outskirts and killed at least one person. Ukrainian President Volodymyr Zelenskyy denounced the "deliberate and cynical missile strikes" against civilian targets as acts of terrorism.

Ukraine's second-largest city of Kharkiv appeared to be without power Sunday night. Cars drove through darkened streets, and the few pedestrians used flashlights or mobile phones to light their way.

In Sunday night's missile attacks by Russia, the Kharkiv and Donetsk regions seemed to bear the brunt. Dnipropetrovsk, Zaporizhzhia and Sumy had only partially lost power, Zelenskyy said. Kharkiv Mayor Igor Terekhov called the power outage "revenge by the Russian aggressor for the successes of our army at the front, in particular, in the Kharkiv region".

Ukrainian officials said Russia hit Kharkiv TEC-5, the country's second-biggest heat and power plant, and Zelenskyy posted video of the Kharkiv power plant on fire.

"Russian terrorists remain terrorists and attack critical infrastructure. No military facilities, only the goal of leaving people without light and heat," he tweeted.?

Russia is frustrated with its poor situation in Eastern Ukraine. Russian military resorted to retreat to prevent its forces from being surrounded. In their retreat, the Russian soldiers have left behind significant numbers of weapons and munitions.

The think tank The Institute for the Study of War (ISW) said, "Geolocated and social media footage confirmed that Ukrainian forces have entered Vovchansk and Velykyi Burluk, severing Russian ground lines of communication (GLOCs) along the T2104 highway and reaching the international border...Combat footage taken in Bilohorivka, Luhansk Oblast, on September 11 confirms that #Ukrainian troops have retaken ground in Luhansk Oblast."

The ISW also said that the Russians fled in haste, leaving behind tanks and other equipment, suggesting that they could not properly organise the retreat.

"Russian troops likely withdrew from the area in great haste, and social media posts show abandoned tanks and other heavy military equipment near Izyum, which indicates that Russian troops failed to organize a coherent retreat," said ISW on Twitter.

Despite Russian strikes on power stations causing outages, Zelenskyy remained defiant. Addressing Russia, he said: "Do you still think you can intimidate, break us, force us to make concessions?...Cold, hunger, darkness and thirst for us are not as scary and deadly as your 'friendship and brotherhood'. But history will put everything in place. And we will be with gas, lights, water and food and without you!"

Later on Sunday evening some power had been restored in some regions.?

The flight of Russian forces in recent days marked the biggest battlefield success for Ukrainian forces since they thwarted a Russian attempt to seize capital Kyiv near the start of the war in February-March. The Kharkiv campaign seemed to take Moscow by surprise.?

(With AP inputs)