Ukraine has accused Russia of attacking civilian infrastructure after strikes on the country's power grid caused widespread blackouts. 

Officials said the bombardment was revenge for a rapid weekend offensive which drove Russia from its main bastion in the Kharkiv region.

They said targets included water facilities and a thermal power station in Kharkiv.

"No military facilities, the goal is to deprive people of light and heat," Ukrainian President Volodymyr Zelenskyy wrote on Twitter late on Sunday.

The US ambassador to Ukraine, Bridget Brink, also denounced the strikes.

"Russia's apparent response to Ukraine liberating cities and villages in the east: sending missiles to attempt to destroy critical civilian infrastructure," she tweeted.

Moscow denies its forces deliberately target civilians.

Mr Zelenskyy has described Ukraine's offensive in the north-east 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 kilometres since the start of this month.

Moscow's almost total silence on the defeat — or any explanation of what happened in north-eastern Ukraine — has provoked some pro-war commentators and Russian nationalists on social media, who have called on President Vladimir Putin to make immediate changes to ensure Russia wins the war.

Russian strikes 'cynical revenge'

Mr Zelenskyy 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.

Ukrainian officials said Russia hit Kharkiv TEC-5, the country's second-biggest heat and power plant.

Kharkiv Mayor Ihor Terekhov described Sunday's attacks as "cynical revenge" for the success of Ukrainian troops, particularly in Kharkiv.

Ukraine's gains are important politically for Mr Zelenskyy as he seeks to keep Europe united behind Ukraine — supplying weapons and money — even as an energy crisis looms this winter after cuts in Russian gas supplies to European customers.

He has said Ukrainian forces would continue to advance.

Kyiv-based military analyst Oleh Zhdanov said the gains could bring a further push into the Luhansk region, which Russia captured 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.

'It's a sign Russia can be defeated'

Defence Minister Oleksii Reznikov said Ukraine needed to secure recaptured territory against a possible Russian counterattack on stretched Ukrainian supply lines. 

He told the Financial Times 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.