An ally of President Vladimir Putin issued a stark new nuclear warning to Ukraine and the West on Tuesday (local time) as Russia began releasing results of referendums it bills as a prelude to it annexing four Ukrainian regions.

Moscow's latest broadside came as European countries rushed to investigate unexplained, major leaks in two Russian natural gas pipelines under the Baltic Sea, which posed risks of explosions and the sinking of any ships that enter the area.

The Kremlin, which has blamed technical problems for earlier cuts in Russian gas supplies to Europe, said it could not rule out sabotage, without saying who was to blame.

Russia's confrontation with the West has driven up global inflation and sharpened energy and food crises in many countries since its Feb. 24 invasion of Ukraine, which was met by tough Western sanctions and Russian retaliatory measures.

Tuesday's nuclear warning by Dmitry Medvedev, deputy chairman of Russia's Security Council, is one of several issued by Putin and his associates in recent weeks.

Analysts say they are designed to deter Ukraine and the West by hinting at a readiness to use tactical nuclear weapons to defend newly annexed territory, where Russian forces have faced strong Ukrainian counteroffensives in recent weeks.

Medvedev's warning differed from earlier ones in that he predicted for the first time that the NATO military alliance would not risk a nuclear war and directly enter the Ukraine war even if Moscow struck Ukraine with nuclear weapons.

"I believe that NATO would not directly interfere in the conflict even in this scenario," Medvedev said in a post on Telegram.

"The demagogues across the ocean and in Europe are not going to die in a nuclear apocalypse."

'SHAM' REFERENDUMS

First partial results from votes cast inside Russia on whether the four Russian-occupied regions of Ukraine should be incorporated into Russia showed overwhelming majorities in favour, Russia's state news agency RIA reported.

It was a widely anticipated announcement after a five-day referendum that Kyiv and the West have denounced as a sham and say they will not recognise. Ukraine urged the EU to impose new punitive sanctions in response to votes it said were carried out at gunpoint in many cases.

Putin said on state TV the votes were designed to protect people from what he has called the persecution of ethnic Russians and Russian-speakers by Ukraine, something Kyiv has denied.

"Saving people in all the territories where this referendum is being held is at the top of our minds and the focus of attention of our entire society and country," Putin said.

He earlier discussed the mobilisation of farmers to fight in Ukraine with officials, the latest step in a campaign he announced last week to support what Moscow calls its "special military operation" after a battlefield reverse this month.

The campaign has sent thousands of Russians rushing to cross Russian borders into neighbouring countries.

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Russian government officials have repeatedly warned they might use nuclear weapons if Kyiv's forces, who control some of the areas Moscow claims, try to take what Russia will soon regard as sovereign territory.

Washington has said it had spelled out privately to Moscow what it described as "catastrophic consequences" for Russia.

Ukrainian presidential adviser Mykhailo Podolyak said Ukraine was preparing for the possibility of a Russian nuclear strike, but the onus was on nuclear-armed states to deter it.

"...Where exactly should we evacuate people in the event of a Russian nuclear strike against Ukraine?" he asked in an interview with Swiss newspaper Blick. "That is why the use of nuclear weapons is a question of global security."

In central Kyiv, music teacher Andrii Liubomyr said he was unfazed by the possibility of a nuclear strike.

"So what? So what? What do we have to fear after February 24? There is nothing more to fear," he told Reuters, referring to the start of the Russian invasion.

Podolyak said that Ukrainians who had helped Russia organise the annexation referendums would face treason charges and at least five years in jail.

"We have lists of names of people who have been involved in some way," he said, adding that Ukrainians who were forced to vote would not be punished. Ukrainians officials have reported ballot boxes being taken door to door and residents being coerced into voting in front of Russian troops.

"I consider this (referendum) illegal and absurd," said 70-year-old Kyiv resident Oleksandr Pylpenko. "I think it’s just a populist move."

None of the provinces are fully under Moscow's control and fighting has been under way along the entire front line, with Ukrainian forces reporting more advances since they routed Russian troops in a fifth province, Kharkiv, earlier this month.

Putin is expected to announce the accession of the occupied regions to the Russian Federation in the coming weeks during an address to parliament on Sept. 30.

The head of the upper house of the Russian parliament said the chamber may consider the incorporation of the four regions into Russia on Oct 4.

'SEVERE FIGHTING'

Ukrainian and Russian forces were locked in heavy fighting in different parts of Ukraine on Tuesday.

Ukrainian President Volodymyr Zelenskiy said the Donetsk region in the east remained his country's -- and Russia's -- top strategic priority, with "particularly severe" fighting engulfing several towns.

Pavlo Kyrylenko, the regional governor, said three civilians had been killed in the past 24 hours.

There were also reported clashes in the Kharkiv region in the northeast -- focus of a Ukrainian counter-offensive this month.

Ukrainian forces in the south continued trying to keep bridges and other river crossings out of action to disrupt supply lines to Russian forces. The air force said it had shot down three Iranian-made drones operated by Russia after an attack on the Mykolaiv region.

Reuters could not immediately verify battlefield reports.