The Kremlin today threatened Britain with ‘tough retaliatory measures’ as President Vladimir Putin said devastating Western sanctions against Russia are verging on a declaration of war, and warned that any country imposing a no-fly zone over Ukraine would be considered to have entered the ongoing conflict.

The threats came as Moscow’s brutal assault on Ukraine saw a mass civilian evacuation from Mariupol derailed when Russian forces ignored a promised ceasefire and continued shelling the southern city.

Britain’s Ministry of Defence (MoD) said Saturday evening that Russia’s proposed ceasefire was likely an attempt to regroup its forces for a new onslaught, while also an attempt to deflect international condemnation.

‘By accusing Ukraine of breaking the agreement, Russia is likely seeking to shift responsibility for current and future civilian casualties in the city,’ the British defence ministry said in an intelligence update.

Russia’s defence ministry accused Ukrainian ‘nationalists’ of preventing civilians from leaving, RIA news agency reported. But Mariupol’s city council said Russia was not observing the ceasefire.

A defiant Putin today called the West’s economic, diplomatic and cultural boycott of the country tantamount to a declaration of war and warned that his regime would consider any third-party declaration of a no-fly zone as ‘participation in the armed conflict’.

In an astonishing display of sabre-rattling, Russian foreign ministry spokeswoman Maria Zakharova warned London: ‘Russia will not forget Britain’s desire to co-operate with ultra-nationalist forces in Ukraine and the supply of British weapons to the Kiev regime’.

She added: ‘The sanctions hysteria in which London plays one of the leading, if not the main, roles, leaves us no choice but to take proportionately tough retaliatory measures. London has made a final choice of open confrontation with Russia.

‘Such a development convinces us once more that Russophobia and the aim to undermine the Russian state are integral elements of Britain’s foreign policy.’

In a dramatic escalation of tensions between Russia and the West last week, Putin announced that Moscow was putting its nuclear deterrence on ‘alert’.

Speaking to a group of female pilots at an Aeroflot training centre near Moscow, the Russian despot reiterated that his war aims are the ‘demilitarisation’ and ‘de-Nazification’ of Ukraine and its neutrality – dismissed as baseless pretexts by Kyiv and her partners including Washington and London.

‘These sanctions that are being imposed are akin to a declaration of war, but thank God it has not come to that‘, Putin jibed.

Talking about the prospect of a NATO state enforcing a no-fly zone over Ukraine, the Russian warmonger continued: ‘That very second, we will view them as participants of the military conflict, and it would not matter what members they are.’

Ukraine’s president Volodymr Zelensky last night railed against the West over its refusal to impose a no-fly zone over his country – an action which would compel third parties such as Britain and the United States to shoot down Russian aircraft that flew in Ukrainian airspace, and turn the war in Ukraine into a full-scale global war.

But as the United States and other NATO members send weapons for Kyiv and more than 1million refugees spill through the continent, the conflict is already drawing in countries far beyond Ukraine’s borders.

In a bitter and emotional speech late on Friday, Zelensky warned that ‘all the people who die from this day forward will also die because of you’.

The comic-turned-wartime leader said NATO ‘has given the green light to the bombing of Ukrainian cities and villages’, adding that ‘the history of Europe will remember this forever’. In a separate video message to anti-war protesters in several European cities, he said: ‘If we fall, you will fall’.

Ukrainian officials on Saturday blamed Russian shelling for breaching a ceasefire arranged in two cities in the country’s south to evacuate more than 200,000 civilians.

The struggle to enforce the ceasefire in the strategic port city Mariupol and Volnovakha showed the fragility of efforts to stop fighting across Ukraine, as the number of people fleeing the country reached 1.4million just 10 days after Russian forces invaded.

Putin accused Ukraine of sabotaging the evacuation and even claimed Ukraine’s leadership was calling into question the future of the country’s statehood, saying that ‘if this happens, it will be entirely on their conscience’.

Ukraine’s military claims that it has killed around 10,000 Russian troops since the invasion on February 24 – far beyond the 498 claimed by Moscow. Kyiv estimates that Russian losses also include 269 tanks, 105 artillery systems, 39 aircraft, 40 helicopters and 409 vehicles.

The Russian defence ministry said on Saturday that its units had opened humanitarian corridors near the two cities encircled by its troops for five hours between 12pm and 5pm Moscow time, Russia’s RIA news agency reported.

In Mariupol, citizens would be allowed to leave during a five-hour window, it quoted the city’s officials as saying. The Russian defence ministry said a broad offensive would then continue in Ukraine, RIA said.

The Ukrainian government said the plan was to evacuate around 200,000 people from Mariupol and 15,000 from Volnovakha, and the Red Cross would be the ceasefire’s guarantor.

The evacuation would have been seen as a prelude to a final assault that, if successful, would see the Russian army push north from occupied Crimea and link up with their forces from the east and take control of Ukraine’s coast on the Sea of Azov.

Since Russia invaded on February 24, Moscow has pummelled Ukrainian cities, with officials reporting hundreds of civilians killed. Europe’s largest atomic power plant has even come under attack sparking fears of a catastrophic nuclear accident. But Russia has so far only seized two key cities, Berdiansk and Kherson on Ukraine’s southern Black Sea coast.

Capturing Mariupol represents a bigger prize for Russian forces as it would deal a severe blow to Ukraine’s maritime access and connect with troops coming from annexed Crimea and the Donbas.

In comments carried on Ukrainian television, Mariupol mayor Vadym Boychenko said thousands of people had gathered for safe passage out of the city and buses were departing when the shelling began.

‘We value the life of every inhabitant of Mariupol and we cannot risk it, so we stopped the evacuation,’ he said.

Before Russia announced the ceasefire, Ukraine had urged Moscow to create humanitarian corridors to allow children, women and older adults to flee the fighting, calling them ‘question number one.

Putin’s lapdog Sergey Lavrov on Saturday said Russia was ready for the third round of talks on that and other issues, but he said that ‘the Ukrainian side, the most interesting side here, it would seem, is constantly making up various pretexts to delay the beginning of another meeting’.