By Pavel Polityuk and Tom Balmforth
KYIV, KHARKIV/Ukraine (Reuters) -Ukrainian forces kept pushing north in the Kharkiv region and advancing to its south and east, Ukraine's army chief said on Sunday, a day after their rapid surge forward drove Russia to abandon its main bastion in the area.
Ukrainian officials accused retreating Russian forces of launching retaliatory attacks on civilian infrastructure, including a thermal power station in Kharkiv, that the authorities in Kyiv said caused widespread blackouts.
"No military facilities, the goal is to deprive people of light & heat," Ukrainian President Volodymyr Zelenskiy wrote on Twitter of the attacks.
Moscow denies its forces deliberately target civilians.
Zelenskiy has described Ukraine's offensive as a potential breakthrough in the six-month-old war, and said the winter could see further territorial gains if Kyiv received more powerful weapons.
In the worst defeat for Moscow's forces since they were repelled from the outskirts of the capital Kyiv in March, thousands of Russian soldiers left behind ammunition and equipment as they fled the city of Izium, which they had used as a logistics hub.
Ukraine's chief commander, General Valeriy Zaluzhnyi, said the armed forces had regained control of more than 3,000 square km (1,158 square miles) since the start of this month.
Moscow's almost total silence on the defeat - or any explanation for what had taken place in northeastern Ukraine - provoked significant anger among some pro-war commentators and Russian nationalists on social media. Some called on Sunday for President Vladimir Putin to make immediate changes to ensure ultimate victory in the war.
Zelenskiy said late on Sunday that Russian attacks caused a total blackout in the Kharkiv and Donetsk regions, and partial blackouts in the Zaporizhzhia, Dnipropetrovsk and Sumy regions.
"They are unable to reconcile themselves to defeats on the battlefield," Dnipropetrovsk Governor Valentyn Reznichenko wrote on Telegram.
Kyrylo Tymoshenko, the deputy head of the president's office, posted an image on Telegram of a power station on fire but added power had been restored in some regions.
Kharkiv Mayor Ihor Terekhov described attacks on infrastructure as "cynical revenge" for the success of Ukrainian troops at the front, particularly in Kharkiv.
Ukraine's gains are important politically for Zelenskiy as he seeks to keep Europe united behind Ukraine - supplying weapons and money - even as an energy crisis looms this winter following cuts in Russian gas supplies to European customers.
Zelenskiy said Ukrainian forces would continue to advance.
"We will not be standing still," he said in a CNN interview recorded on Friday in Kyiv. "We will be slowly, gradually moving forward."
'SNOWBALL ROLLING DOWN A HILL'
Defence Minister Oleksii Reznikov said Ukraine needed to secure retaken territory against a possible Russian counterattack on stretched Ukrainian supply lines. He told the Financial Times that Ukrainian forces could be encircled by fresh Russian troops if they advanced too far.
But he said the offensive had gone far better than expected, describing it as a "snowball rolling down a hill".
"It's a sign that Russia can be defeated," he said.
Kyiv-based military analyst Oleh Zhdanov said the gains could bring a further push into Luhansk region, whose capture Russia claimed at the beginning of July.
"If you look at the map, it is logical to assume that the offensive will develop in the direction of Svatove - Starobelsk, and Sievierodonetsk - Lysychansk," he said.
The head of Russia's administration in Kharkiv told residents to evacuate the province and flee to Russia, TASS reported on Saturday. Witnesses described traffic jams with people leaving Russian-held territory.
Washington appeared to take a cautious public posture, with the Pentagon referring Reuters to Defense Secretary Lloyd Austin's remarks on Thursday about Kyiv's "encouraging" battlefield successes.
Britain's defence ministry said on Sunday that fighting continued around Izium and the city of Kupiansk, the sole rail hub supplying Russia's front line across northeastern Ukraine, which has been retaken by Ukraine's forces.
NUCLEAR REACTOR SHUTS DOWN
As the war entered its 200th day, Ukraine on Sunday shut down the last operating reactor at Europe's biggest nuclear power plant to guard against a catastrophe as fighting rages nearby.
Russia and Ukraine accuse each other of shelling around the Russian-held Zaporizhzhia plant, risking a release of radiation.
The International Atomic Energy Agency said a backup power line to the plant had been restored, providing the external electricity it needed to carry out the shutdown while defending against the risk of a meltdown.
French President Emmanuel Macron told Putin in a phone call on Sunday that the plant's occupation by Russian troops is the reason why its security is compromised, the French presidency said. Putin blamed Ukrainian forces, according to a Kremlin statement.
(Reporting by Reuters reporters; Writing by Tomasz Janowski, William Maclean and Phil Stewart; Editing by Andrew Heavens, Frances Kerry and Daniel Wallis)