Ukrainian President Volodymyr Zelensky speaks in Kyiv. File photo

European leaders on Sunday sought to ease the impact of high energy prices across the continent, after Ukrainian President Volodymyr Zelensky warned of a difficult winter, even as he reported progress in a counter-offensive against Russian troops.

Energy problems plagued Ukraine and Europe as much of the Russian-occupied region that's home to a largely crippled nuclear power plant was reported temporarily in blackout on Sunday.

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Zelensky thanked his forces in his nightly address on Sunday for taking two settlements in the south and a third in the east, as well as additional territory in the east, saying he had received "good reports" from his military commanders and head of intelligence.

Meanwhile, the fighting in Ukraine and related disputes over pipelines lie behind the electricity and natural gas shortfalls that have worsened as Russia's war in Ukraine, which began on Feb. 24, grinds on for a seventh month.

President Volodymyr Zelensky reports progress in a counter-offensive against Russian troops.

Kyrylo Tymoshenko, deputy head of the president's office, earlier on Sunday posted an image of soldiers raising the Ukrainian flag over a village he labeled as being in the southern area that is the main focus of the counter-offensive.

"Vysokopillya. Kherson region. Ukraine. Today," Tymoshenko wrote in a Facebook post over a photo of three soldiers on rooftops, one of them fixing a Ukrainian flag to a post.

Ukraine began a counter-offensive last week targeting the south, particularly the Kherson region, which Russia seized early in the conflict.

Zelensky's remarks came a day after he warned Europeans that Russia was preparing "a decisive energy blow" during the cold months ahead.

Ukraine accuses Russia of weaponising energy supplies.

Moscow has cited Western sanctions and technical issues for the energy disruptions. European countries who have backed Kyiv with diplomatic and military support have accused Russia of weaponising energy supplies.

Some analysts say the shortages and a surge in living costs as winter approaches risk sapping Western support for Kyiv as governments try to deal with disgruntled populations.

Separately, the US embassy in Moscow said John Sullivan, the ambassador since his appointment by former President Donald Trump in 2019, had left his post and was retiring from the diplomatic service. A State Department official said Sullivan had served a typical tour length.

Last week Moscow said it would keep the Nord Stream 1 pipeline, its main gas channel to Germany, closed and G7 countries announced a planned price cap on Russian oil exports.

German Chancellor Olaf Scholz speaks to the media. File photo

The Kremlin said it would stop selling oil to any countries that implemented the cap.

German Chancellor Olaf Scholz said on Sunday his government had been planning for a total halt in gas deliveries in December, promising measures to lower prices and tie social benefits to inflation.

"Russia is no longer a reliable energy partner," Scholz told a news conference in Berlin.

In response, former Russian President Dmitry Medvedev accused Germany of being an enemy of Russia. "In other words, it has declared a hybrid war on Russia," he said.

On Sunday, Finland and Sweden announced plans to offer billions of dollars to power companies to avert the threat of insolvency amid the crisis.