Boris Johnson said Russian President Vladimir Putin’s invasion of Ukraine is a “catastrophe for our continent”, as he called on world leaders to meet and plan a response.
The Prime Minister tweeted his remarks after chairing an urgent Cobra emergency committee meeting on Thursday morning.
He is due to make a televised address to the nation at midday before speaking in the Commons in the evening.
Mr Johnson said: “This is a catastrophe for our continent.
“I will make an address to the nation this morning on the Russian invasion of Ukraine.
“I will also speak to fellow G7 leaders and I am calling for an urgent meeting of all Nato leaders as soon as possible.”
Follow the latest from Mr Johnson’s address in our live blog.
Putin accused of ‘bringing war back to Europe’
Foreign Secretary Liz Truss summoned Russian ambassador to the UK Andrei Kelin, for the second time this week, to explain the “illegal, unprovoked invasion of Ukraine”.
Ms Truss said: “We will be imposing severe sanctions and rallying countries in support of Ukraine.”
It came as Ukraine President Volodymyr Zelensky declared martial law and urged people to stay indoors, amid reports of missile strikes and explosions across the country.
Ursula von der Leyen, president of the European Commission, accused Mr Putin of “bringing war back to Europe”.
Mr Zelensky called on Ukrainian nationals to volunteer for the resistance effort.
He said: “We will give weapons to anyone who wants to defend the country. Be ready to support Ukraine in the squares of our cities.
“We will lift sanctions on all citizens of Ukraine who are ready to defend our country as part of territorial defence with weapons in hands.
“We have severed diplomatic relations with Russia. For all those who have not yet lost their conscience in Russia, it is time to go out and protest against the war with Ukraine.”
I am appalled by the horrific events in Ukraine and I have spoken to President Zelenskyy to discuss next steps.
President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine.
The UK and our allies will respond decisively.
— Boris Johnson (@BorisJohnson) February 24, 2022
UK will respond with ‘unprecedented’ sanctions
Russia said it has only targeted Ukrainian air bases and other military assets, not populated areas.
But in a televised address, Mr Putin warned other countries that any attempt to interfere with the Russian action would lead to “consequences they have never seen”.
He said Russia does not intend to occupy Ukraine, and claimed responsibility for bloodshed lies with the Ukrainian “regime”.
Foreign Office minister James Cleverly described the comments as “bully” tactics and said the UK will respond with “unprecedented” sanctions “to punish the appalling decision” by Russia’s leader.
A Downing Street spokesman said Mr Johnson had assured Mr Zelensky during an early-morning telephone call that the West will “not stand by as President Putin waged his campaign against the Ukrainian people”.
Explosions could be heard in the Ukrainian capital, Kyiv, shortly after Mr Putin’s address, while explosions were also reported in the cities of Odesa and Kharkiv.
Elsewhere, footage appeared to show queues of people fleeing their homes, and Russian military crossing the border into Ukraine.
Mr Putin said the action was a response to threats from Ukraine.
And in a warning to the international community during his address on Russian television, he said: “Whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history.”
Foreign Office minister Mr Cleverly rubbished his explanation.
He told BBC Breakfast: “If Vladimir Putin thinks that he can scare the international community away from supporting Ukrainians in defence of their homeland, he is absolutely wrong on that and should be under absolutely no illusion that we will continue to support the Ukrainian government and Ukrainian people.”