In a video message released on Friday night, the prime minister also spoke Ukrainian urging an end to the conflict.

Mr Johnson's latest comments after Kyiv was hit by airstrikes and fighting got closer to the capital.

In the two-minute video posted on Twitter, Mr Johnson said: "The scenes unfolding in the streets and fields of Ukraine are nothing short of a tragedy.

Live updates as Putin authorises military operation

"Brave young soldiers and innocent civilians are being cut down, tanks are rumbling through towns and cities, missiles raining indiscriminately from the skies.

"It's a generation or more since we witnessed such bloodshed in Europe. We hoped we would never have to see such sights again.

"The people of the United Kingdom stand with our Ukrainian brothers and sisters in the face of this unjustifiable assault on your homeland.

"We salute the fierce bravery and patriotism of your government, your military and your people."

The prime minister added that "Putin's actions are leading to complete isolation for Russia".

Key developments:

• Russian invasion reaches Ukraine's capital Kyiv
• Putin urges Ukrainian military to overthrow country's leaders
• Russia kicked out of Eurovision
• Ukraine troops tell Russian warship 'go f*** yourself' before being killed
• EU and US place sanctions on Putin and Lavrov
• NATO bolstering eastern flank

UK to sanction Putin and Lavrov personally

Earlier on Friday, it was announced that the UK will impose direct sanctions on Vladimir Putin and Russian Foreign Minister Sergei Lavrov imminently.

At a virtual meeting with world leaders, the prime minister said his government will personally sanction the Russian president and Mr Lavrov over their "revanchist mission" to overturn the post-Cold War order.

Mr Johnson also said that "immediate action" must be taken to ban Russia from the SWIFT global interbank messaging system that helps facilitate payments.to "inflict maximum pain" on President Putin.

Foreign Secretary Liz Truss said the government is introducing the sanctions against President Putin and Mr Lavrov "in response to Russia's illegal and unprovoked invasion of Ukraine".

"We will not stop inflicting economic pain on the Kremlin until Ukrainian sovereignty is restored," she added.

Transport Secretary Grant Shapps also confirmed he has extended a flight ban meaning "no Russian private jet can fly in UK airspace, or touchdown - effective immediately".

US and EU also sanction Russian president

The EU has already sanctioned President Putin and his foreign minister after similarly being accused by Ukraine's president of failing to react hard and fast enough against Russian aggression.

Later on Friday evening, the US also announced its intention to place individual sanctions on the pair.

Read more: Four ways the Russian invasion of Ukraine could end

Ukrainian President Volodymyr Zelenskyy had called on both the UK and EU to strengthen their package of measures hitting oligarchs supporting the Putin regime and freezing Russian bank assets.

PM: Invasion 'Euro-Atlantic crisis with global consequences'

Mr Johnson also warned fellow NATO members that "the Russian president's ambitions might not stop there and that this was a Euro-Atlantic crisis with global consequences", a Number 10 spokesperson said.

Less than an hour earlier, Western officials warned that Russia could use thermobaric weapons against Ukrainian forces if they continue to hold up their advance.

Explainer: What is the SWIFT global payments system?

At present, Ukrainian soldiers are believed to be holding up the Russian advance on Kyiv - with heavy armour still 50km away from the capital.

Earlier this morning, Defence Secretary Ben Wallace told Sky News that more than 450 Russian troops had been killed, as a fierce resistance meant Moscow "failed" on the main objective on the first day of fighting.

Mr Wallace said it was the UK government's view that President Putin intends "to invade the whole of Ukraine".

Read more: Putin urges Ukrainian military to overthrow country's leaders

PM hits Russia with 'largest set of sanctions every imposed' by the UK

Mr Johnson on Thursday hit Russia with what he described as the "largest set of sanctions ever imposed anywhere by the UK government", after the attack on Ukraine.

Read more: Boris Johnson unleashes UK's 'largest set of sanctions ever' on Russia

Making a statement in the Commons, Mr Johnson added that there are also plans to introduce legislation that would limit the amount of deposits Russian nationals will be able to hold in UK bank accounts.

The prime minister spoke with leaders of the Joint Expeditionary Force today - made up of the UK, Denmark, Finland, Estonia, Iceland, Latvia, Lithuania, the Netherlands, Sweden and Norway.

Explainer: Who are the oligarchs with Kremlin ties targeted in UK's new sanctions against Russia?

Meanwhile, Russia's foreign minister Mr Lavrov said Moscow would be ready to hold talks with Kyiv but only once Ukraine's military had laid down its arms.

He also said the Kremlin did not want "neo-Nazis" to govern Ukraine.