Western leaders fear Vladimir Putin is planning a huge escalation of Russia's faltering invasion of Ukraine with a deadly nuclear strike - as UK officials believe he now realises war was a 'colossal mistake'

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Western leaders fear Vladimir Putin is planning a huge escalation of Russia's faltering invasion with a deadly nuclear strike or an attack on European interests.

British and American spies believe there is 'credible intelligence' to suggest the Russian president is preparing an attack that will turn the war effort in his favour after coming to the realisation that the war was 'a colossal mistake'.

It comes as the US Embassy in Moscow is urging citizens to flee Russia amid fears that dual nationals could be drafted into its war efforts, The Mirror reports.

Embassy officials have said Russia may 'refuse to acknowledge US nationals' dual citizenship', deny their access to consular assistance and 'prevent their departure from Russia'.

The embassy also warned that leaving Russia has become increasingly difficult with flights filling and Putin's troops shutting down borders. 

Putin is understood to be planning land-grab of four regions said to have voted overwhelmingly to join Russia after sham referendums in which residents were marched to the ballot box at gunpoint.

Hastily arranged votes had taken place over five days in the four areas - Donetsk, Luhansk, Zaporizhzhia and Kherson - that make up about 15 per cent of Ukrainian territory.

But there is concern that Putin is planning a nuclear attack west of the regions that would create a radiation dead zone and put a halt to Ukraine's so far successful counter-offensives.

Another fear is that Russia is preparing a strike on western interests such as maritime communications cables.

 British and American spies believe there is 'credible intelligence' to suggest the Russian president is preparing a huge escalation

Members of an election commission count ballots after voting during a sham referendum at a polling station in Donetsk

Russians are seen attempting to leave their country to avoid a military call-up for the Russia-Ukraine war at the Kazbegi border crossing in the Kazbegi municipality of Stepantsminda, Georgia

Officials in the UK believe Putin, as well as Russian population, now understand the war was a 'colossal mistake.

They also fear the partial mobilisation of 300,000 conscripts may be con to deceive the West into concentrating on Ukraine while he tried to leverage against support for Kyiv.

It comes amid fears that Kremlin frogmen are responsible for the 'deliberate' sabotage of the Nord Stream gas supply pipeline in the Baltic Sea that has sent energy prices soaring.

A security source told The Mirror that the leaks could act as a warning of similar assaults on communications cables.

They added: 'Putin has almost run out of options which makes a tactical nuclear strike possible west of the four regions he is claiming.

'It is possible he believes this will be enough as a show of intent to get the west to back off, create a dead zone to halt the Ukrainian counter-offensive and claim victory in the east.

'But another possibility is an attack on the communication cables between the UK and America, many of which are European links to the States.'

Damaging internet communication cables could be 'very damaging' the world's financial markets. 

Putin last week announced that he would mobilize 300,000 military-aged men to fight Ukraine - setting off chaos in recruitment centres, airports and at the borders. 

But Russian troops have been deployed near the border with Georgia this week as the Kremlin tries to stamp out rumours that martial law will be introduced and people banned from leaving in order to avoid being conscripted into the army.

Footage geolocated to the small town of Chmi, on the main highway from Russia into Georgia's South Ossetia region, shows at least six soldiers sitting on top of an armoured vehicle passing a queue of cars in the direction of the crossing.

Troops arrived to set up a checkpoint, sources told the Russia's RBC network, with the FSB saying they were sent to stop people storming the crossing after miles-long queues of men trying to dodge the draft built up.

Meanwhile, Western leaders have blamed Russia for the ‘deliberate’ sabotage after the Nordsteam pipeline suffered 'unprecedented' damage off Sweden.

The Kremlin has denied any involvement in the explosions, dismissing the suggestion as 'stupid '. But Russia does have the capacity to carry out such an attack.

Putin commands the world's largest fleet of spy submarines including one - the Belgorod - that was specifically designed to attack undersea cables, and has easy access to the Baltic via Kaliningrad and St Petersberg, where its newly-developed fleet of underwater drones is also based.

As Dr Stepan Stepanenko, research fellow at the Henry Jackson Society, told MailOnline: 'At first glance, this act equates to cutting one's nose off to spite the face [but] to understand why Russia would do this, we have to look at it in the context of the sham referenda taking place in Ukraine.

'Putin is fast running out of options, he needs to end the conflict with a tangible territorial gain before Russians twig what the rest of the world already knows: He lost the war.

'By conducting these sham referenda and moving the border to make these regions Russian territory, he is backing the West into an ultimatum; liberate these regions and you are directly attacking Russia.

'In this context, if Nord Stream was indeed targeted by Moscow then we can't avoid seeing it as Putin preparing for a global war, in which supplying gas to Europe would be directly aiding his enemy.'