Amid an international outcry at Moscow's actions, NATO ambassadors said in a statement after emergency talks that "we have increased the readiness of our forces to respond to all contingencies".

Explosions near major Ukrainian cities including the capital Kyiv - follow live updates

The alliance has agreed to strengthen its land, sea and air forces on its eastern flank near Ukraine - but NATO will not launch any military action in support of Ukraine.

European Commission President Ursula von der Leyen said EU leaders would be asked to approve "massive and targeted sanctions" against Russia.

The new EU sanctions would be aimed at "strategic sectors" of the Russian economy by blocking access to technologies and key markets, Ms von der Leyen said.

"We will weaken Russia's economic base and its capacity to modernise," she added.

"In addition we will freeze Russian assets in the EU and stop the access of Russian banks to European financial markets."

Ms Von der Leyen said the fresh sanctions - aimed at taking a "heavy toll on the Kremlin's interests and their ability to finance war" - would be "closely aligned" with action being taken by countries such as the US, UK, Canada, Japan and Australia.

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She accused the Russian president of "trying to turn the clock back to the times of the Russian Empire" with his "barbaric attack" on Ukraine.

"It is President Putin who is bringing war back to Europe and in these dark hours the EU and its people stand by Ukraine and its people," she said.

Josep Borrell, the EU's foreign policy chief, said the bloc's leaders would adopt "the harshest package of sanctions we have ever implemented".

The UK has also promised an "unprecedented level of sanctions" against Russia following its fresh aggression on its neighbour, with G7 leaders set to speak on Thursday.

Prime Minister Boris Johnson accused Mr Putin of choosing a "path of bloodshed and destruction" after explosions were heard in Ukrainian cities in the early morning.

Mr Johnson chaired a meeting of the government's emergency COBRA committee from 7.30am and will make a televised address to the nation later this morning.

The PM, who is calling for an urgent meeting of NATO leaders, is also due to make a statement to the House of Commons around 5pm to outline new sanctions on Russia.

US President Joe Biden described Mr Putin's decision to begin military operations as an "unprovoked and unjustified attack", declaring: "The world will hold Russia accountable."

Mr Biden added: "The prayers of the entire world are with the people of Ukraine tonight."

Mr Putin appeared on state TV in Moscow to announce the operation was going ahead as the UN Security Council was meeting in New York to urge his country not to invade Ukraine.

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António Guterres, the UN secretary-general, said the war "doesn't make any sense" and could cause a "level of suffering not seen in Europe since the Balkan crisis".

Jens Stoltenberg, the NATO secretary-general, said Russia's actions were a "grave breach of international law" and that allies would meet to address the "renewed aggression".

A NATO statement released later on Thursday morning said: "We urge Russia in the strongest terms to turn back from the path of violence and aggression it has chosen.

"Russia's leaders must bear full responsibility for the consequences of their actions. Russia will pay a very heavy economic and political price."

German Chancellor Olaf Scholz said the Russian attack on Ukraine is a "blatant" breach of international law.

French President Emmanuel Macron expressed solidarity with Ukraine and urged Russia to stop military action.

Australian Prime Minister Scott Morrison condemned the "brutal" and "unprovoked" attack and said his country is considering sanctions against 300 members of the Russian parliament.

Canadian Prime Minister Justin Trudeau said his country condemned the Russian attack "in the strongest possible terms" and called for the immediate cessation of "all hostile and provocative actions against Ukraine".

He added: "These reckless and dangerous acts will not go unpunished."

Japanese Prime Minister Fumio Kishida said: "This Russian invasion stands to put at risk the basic principle of international order that forbids one-sided action of force in an attempt to change the status quo."

Labour leader Sir Keir Starmer called for the "hardest possible sanctions" to be taken against "all those linked to Putin".

"There will be dark days ahead," he said. "But Putin will learn the same lesson as Europe's tyrants of the last century: that the resolve of the world is harder than he imagines and the desire for liberty burns stronger than ever."

The UK's ambassador to the UN, Dame Barbara Woodward, said it was a "grave day for Ukraine and for the principles of the United Nations".