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:

US President Joe Biden 

"The prayers of the entire world are with the people of Ukraine tonight as they suffer an unprovoked and unjustified attack by Russian military forces," 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.

Ukrainian President Volodymyr Zelensky 

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.

China 

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.

UN chief Antonio Guterres 

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 conflict must stop now," added the UN chief, who said it was the "saddest day" of his tenure.

NATO head Jens Stoltenberg 

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."

NATO ambassadors were to hold an emergency meeting to discuss the attack.

British Prime Minister Boris Johnson

"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."

EU chiefs 

"In these dark hours, our thoughts are with Ukraine and the innocent women, men and children as they face this unprovoked attack and fear for their lives," European Union chiefs Ursula von der Leyen and Charles Michel said on Twitter. 

"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.

German Chancellor Olaf Scholz 

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

Putin is "endangering the lives of countless innocent people in Ukraine... (and) jeopardising peace in our continent," Scholz said.

French President Emmanuel Macron 

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" when they meet to agree sanctions later on Thursday, after Moscow inflicted "the most significant damage on peace and stability in Europe for decades."

Canadian Prime Minister Justin Trudeau 

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.

OSCE 

The Organization for Security and Co-operation in Europe, of which Russia is a member, denounced the move in a statement. 

It said the attack "puts the lives of millions of people at grave risk and is a gross breach of international law and Russia's commitments."

Japanese Prime Minister Fumio Kishida 

"The latest Russian invasion shakes the foundation of the international order, which does not permit unilateral attempts to change the status quo," Japan's leader said after a meeting of his national security council.

- Italian Prime Minister Mario Draghi -

The leader of G7 member Italy's government called Russia's invasion "unjustified and unjustifiable."

"Italy is close to the Ukrainian people and institutions in this dramatic moment," Draghi said in a statement.

Turkey 

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.