Ukrainian forces reported battlefield gains today in a counterattack that could signal a shift in the momentum of the war.

It comes as Kyiv shut gas flows on a route through Russian-held territory - raising the spectre of an energy crisis in Europe.

In Vilhivka, a village east of Kharkiv held by Ukrainian forces, the thump of near constant artillery and swoosh of multiple rocket launchers could be heard from fighting at the front.

That front has now pushed substantially further east, where Ukraine has been trying to capture the banks of the Donets river and threaten Russian supply lines on the far side.

Following days of advances, Ukrainian forces were within several kilometres of the Russian border in the Kharkiv region this morning, one Ukrainian military source told Reuters.

East of Vilhivka, Ukrainian forces seemed to be in control of the village of Rubizhne, on the banks of the Donets, after what appear to be the fastest advances since April when Ukraine pushed Russian forces back from Kyiv and the country's north.

"It is burned out, just like all Russian tanks," a Ukrainian soldier told Reuters outside the village next to the ruins of one Russian tank. "The weapons are helping a lot, the anti-tank ones."

Members of the Ukrainian military receive medical treatment at a frontline field hospital in Popasna

Kyiv has so far confirmed few details about its advance through the Kharkiv region.

"We are having successes in the Kharkiv direction, where we are steadily pushing back the enemy and liberating population centres," Brigadier General Oleksiy Hromov, Deputy Chief of the Main Operations Directorate of Ukraine's General Staff, told a briefing, providing no specifics.

President Volodymyr Zelensky said successes were putting Ukraine's second largest city - under constant bombardment since the war's early days - beyond the range of Russian artillery.

But he cautioned Ukrainians against raising their expectations too high yet.

"We should not create an atmosphere of excessive moral pressure, where victories are expected weekly and even daily," he said in an overnight video address.

In Vilhivka, the advance had made it possible for residents to venture back to comb through the wreckage of their homes. Although the village itself had been recaptured by Ukrainian forces weeks ago, the frontline was only now far enough away to make it safe to return.

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Today's separate move by Ukraine to cut off Russian gas supplies through territory held by Russian-backed separatists was one of the first times gas shipments to Europe have been directly affected by the conflict.

Gas flows from Russia's export monopoly Gazprom to Europevia Ukraine fell by a quarter after Kyiv said it was forced to halt all flows from the Sokhranovka transit point in southern Russia, because Russian-backed separatists were siphoning supplies.

Should the supply cut-back persist, it would be the most direct impact so far on European energy markets of the war that the Kremlin calls a "special military operation".

Residents queue to get water in Mariupol

Apart from the east, Russia has seized a swathe of southern Ukraine, where Kyiv and its Western allies say they believe Moscow intends to organise a fake referendum on independence or annexation to make its occupation permanent.

The Kremlin said today it was up to residents living in the Russian-occupied Kherson region to decide whether they wanted to join Russia, but any such decision must have a clear legal basis.

Earlier, TASS news agency quoted an official in the Russian-controlled administration as saying the region planned to ask President Vladimir Putin to incorporate it into Russia.

84-year-old Nadiia reacts as she recalls the Russian occupation her village of Sloboda-Kukharivska

Ukraine's general staff said Moscow was trying to capture the steelworks. The Azov Regiment holed up inside it said Russia was bombing from the air and trying to storm it.

"Azovstal is on fire again after the bombing. If there is hell on earth, it is there," wrote an aide to Mariupol's Mayor Vadym Boichenko, who has left the city.

Kyiv says it is likely that tens of thousands of people have been killed in Mariupol. Ukrainian authorities say between 150,000 and 170,000 of the city's 400,000 residents are still living there amid the Russian-occupied ruins.

"Without medicine and medical care, the restoration of the water supply and proper sewerage in the city, epidemics will erupt," said the mayor.

"Today, the majority of the current population is old and sick. Without proper conditions, mortality among vulnerable groups will increase exponentially."