World leaders on Thursday condemned the Russian invasion of Ukraine, with Western capitals vowing to escalate sanctions against Moscow while the head of the United Nations demanded the conflict end immediately.

Key reactions:

"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack," Biden said shortly after the operation began.

He warned "Russia alone is responsible for the death and destruction this attack will bring."

"The world will hold Russia accountable," he declared.

Zelensky compared Russia's invasion of his country to Nazi Germany's military campaigns during World War II.

"Russia has attacked Ukraine in a cowardly and suicidal way, like Nazi Germany did during World War II," Zelensky said in an online briefing.

In a later video address, he said a "new iron curtain" was "lowering and closing Russia off from the civilised world".

Foreign Minister Wang Yi told his Russian counterpart Sergei Lavrov in a phone call that China understands "Russia's reasonable concerns on security issues", China's foreign ministry said.

The foreign ministry repeatedly declined to call the attack an "invasion" at a press briefing on Thursday.

Guterres made a direct and personal plea to Russian President Vladimir Putin after an emergency Security Council session, urging him to stop the attack "in the name of humanity."

"Do not allow to start in Europe what could be the worst war since the beginning of the century," he said.

The UN chief said it was the "saddest day" of his tenure.

The Atlantic alliance's secretary general said Russia had "chosen the path of aggression against a sovereign and independent country".

The attack "puts at risk countless civilian lives", Stoltenberg said in a statement, describing it as a "grave breach of international law, and a serious threat to Euro-Atlantic security."

"I am appalled by the horrific events in Ukraine and I have spoken to President Zelensky to discuss next steps," the British leader tweeted.

In an address to the nation, Johnson later called Putin a "dictator" facing "massive" sanctions for his "attack on democracy and freedom in east Europe and around the world".

Girls hold their mobile phone as they take refuge in a metro station in Kyiv as air raid sirens rang out across the city
Girls hold their mobile phone as they take refuge in a metro station in Kyiv as air raid sirens rang out across the city AFP / Daniel LEAL

"We will hold the Kremlin accountable," they added.

Foreign policy chief Josep Borrell said Russia faced "unprecedented isolation" and would face the "harshest sanctions" the EU has ever imposed.

"This is not a question of blocs. This is not a question of diplomatic power games. It's a matter of life and death," he said.

The German leader lashed out at an "unscrupulous act" and spoke to Zelensky to express his country's "full solidarity".

"This is an attempt to forcibly shift borders within Europe, perhaps even to wipe an entire country off the world map," Scholz said.

Macron said Russia's "act of war" was a "turning point in the history of Europe" with "deep and lasting consequences for our lives" in an address to the nation.

He said the G7, NATO and the European Union would be "without weakness" in agreeing sanctions, after Moscow inflicted "the most significant damage on peace and stability in Europe for decades".

Trudeau condemned Russia's "egregious" and "unprovoked" attack on Ukraine as "a clear further violation of Ukraine's sovereignty and territorial integrity" and of Russia's obligations under international law.

"These reckless and dangerous acts will not go unpunished," he said in a statement.

The Group of Seven most industrialised nations condemned Russia's invasion of Ukraine and said it "fundamentally changed the Euro-Atlantic security situation".

"President Putin has re-introduced war to the European continent. He has put himself on the wrong side of history," G7 leaders said in a statement.

The museum at the site of the former Nazi death camp, a symbol of the Holocaust, said Putin's "insane pseudo-imperial megalomania" had led to an "act of barbarity" that will be "judged by history".

Zeman -- who had previously fostered close ties with Putin -- called the Russian president a "madman", without naming him.

"A couple days ago, I said the Russians weren't crazy and wouldn't attack Ukraine. I admit I was wrong," Zeman said in a televised speech.

Turkey, a NATO member with friendly ties with Ukraine and Russia, said the invasion was "unjust and unlawful" in a foreign ministry statement.

President Recep Tayyip Erdogan denounced Moscow's move as a "heavy blow" to regional peace and stability.