Russian gas began flowing to Europe via a major pipeline on Thursday after a 10-day pause but fears of broader supply cuts remained and Russian forces in Ukraine were seen eyeing the capture of the country's second biggest power plant.

Russian troops shelled cities across eastern and southern Ukraine, Ukrainian officials said, and hit two schools as Moscow's forces carried out limited ground operations in preparation for what is seen as a wider offensive.

Reuters was unable to immediately verify Ukrainian assertions about Russian shelling, which they say has been intense for several weeks, and it was not immediately clear if anyone had been hurt in the strikes on the schools.

The resumption of gas flows via the Nord Stream 1 pipeline to Germany ended a nerve-jangling 10 days for Europe in which politicians expressed concern Russia might not restart them at a time when alternative energy supplies are tight and prices high.

The pipeline has traditionally carried more than one third of Russia's gas exports to Europe but was operating at only 40% of its capacity after Kremlin-controlled Gazprom cut gas exports in a row over the repair of a turbine.

"In view of the missing 60% (capacity) and the political instability, there is no reason yet to give the all-clear," Klaus Mueller, president of Germany's network regulator, wrote on Twitter.

European Union Commission President Ursula von der Leyen has accused Russia of trying to blackmail Europe by using energy as a weapon, something Moscow, which is unable to swiftly redirect all of its gas to other markets, has denied.

Moscow has criticised EU and U.S. sanctions on Russia over its Feb. 24 invasion of Ukraine and military help to Kyiv, saying it had to undertake what it calls a "special military operation" to prevent NATO using Ukraine to threaten Russia.

Ukraine says it needs the weapons to defend itself against what it and the West cast as an unprovoked imperial-style war of aggression designed to steal its land and erase its national identity.

INTENSE SHELLING

The Ukrainian military reported heavy and sometimes fatal Russian shelling in the east and south of the country amid what its said were largely failed attempts by Russian ground forces to advance in the eastern Donetsk region.

Donetsk regional governor Pavlo Kyrylenko said Russian missile strikes had destroyed two schools in the cities of Kramatorsk and Kostiantynivka and had also hit the city of Bakhmut but there was no information yet on casualties.

"Russia is intentionally destroying our cities and towns. Do not expose yourself to danger - evacuate," he wrote on Telegram.

Russia says it does not deliberately target civilians and uses high precision weapons to degrade Ukrainian military targets, but the war has flattened cities, particularly in Russian-speaking areas in the east and southeast of Ukraine.

The mayor of Kharkiv, Igor Terekhov, in his Telegram channel said that one of the most densely populated areas of the city was being shelled and asked people not to leave shelters. Oleh Synehubov, governor of the Kharkiv region, said two people had been killed and 19 wounded, four of them seriously.

Vitaly Kim, governor of the southern Mykolaiv region, said the region had been targeted with seven S-300 missiles. One person had been wounded, he said, and infrastructure, energy facilities and storage facilities damaged.

Multiple blasts were also heard in the Russian-controlled southern region of Kherson overnight and into Thursday, Russian news agency TASS reported.

Reuters could not independently verify the reports.

HEAVY LOSSES

CIA Director William Burns said on Wednesday that the United States estimated that Russian casualties in Ukraine had so far reached around 15,000 killed and perhaps 45,000 wounded and that Ukraine has suffered what he called significant losses too.

Russia classifies military deaths as state secrets even in times of peace and has not updated its official casualty figures frequently during the war.

Scotching persistent speculation that Putin may be suffering from health problems, Burns also said that the Kremlin chief was healthy as far as he knew.

British military intelligence said on Thursday that Russian forces were likely closing in on Ukraine's second biggest power plant at Vuhlehirska, 50 km (31 miles) north-east of Donetsk.

"Russia is prioritising the capture of critical national infrastructure, such as power plants," the ministry, which supports Ukrainian forces, said in a regular bulletin.

It said taking the power plant, a Soviet-era coal-fired facility, was also probably part of Russia's attempt to regain momentum as it tried to advance towards the key cities of Kramatorsk and Sloviansk in eastern Ukraine.

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