Nato on Thursday ordered its military commanders to intensify preparations to defend its allied territory after Russia launched military operations in Ukraine, putting hundreds of warplanes and ships on alert and agreeing to increase troop numbers on its eastern flank.

"Peace on our continent has been shattered," Nato Secretary-General Jens Stoltenberg told a news conference. "Russia is using force to try to rewrite history, and deny Ukraine its free and independent path."

As the conflict intensifies, here's what world leaders are saying:

United States of America

"President Vladimir Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," said US President Joe Biden.

"I will be meeting with the leaders of the G7, and the United States and our allies and partners will be imposing severe sanctions on Russia."

United Kingdom

British Prime Minister Boris Johnson called Putin a “dictator” who now faced “massive” Western sanctions.

“We cannot and will not just look away,” Johnson said in a televised address to the nation, after phoning Ukraine's President Volodymyr Zelensky just after Russian forces moved in.

Ukraine can be assured of continued UK support given that “our worst fears have now come true and all our warnings have proved tragically accurate”, the prime minister said.

Ahead of an emergency virtual meeting of G7 leaders, Johnson said the West “will agree a massive package of economic sanctions designed in time to hobble the Russian economy”.

“Diplomatically, politically, economically — and eventually, militarily — this hideous and barbaric venture of Vladimir Putin must end in failure.”

France

French President Emmanuel Macron, who had laboured until the last minute for a diplomatic solution, said France "firmly condemns Russia’s decision to wage war” and promised support for Ukraine.

“Russia must end its military operations immediately,” he said, and also spoke by phone to Zelenskyy, who demanded “united European support” for Ukraine, according to a statement from the French presidency.

Macron said France is “working with its partners and allies to end the war”.

Romania

Romanian President Klaus Iohannis said that Russia “chose the reprehensible and completely illegal path of massive armed violence against an independent and sovereign state".

Iohannis said that Romania, a country of about 19.5 million people, is ready to deal with economic and humanitarian consequences that the conflict could generate. Romania borders Ukraine and is a member of Nato and the European Union.

He stressed that Romania will not be drawn into the military conflict in Ukraine and said Romanian authorities will take “absolutely all the necessary measures” to ensure the safety of the country’s citizens.

Prague

Czech President Milos Zeman, who has been a leading pro-Russian voice among European Union leaders, condemned Russia’s announcement as “an unprovoked act of aggression".

Zeman said in an address to the nation that “Russia has committed a crime against peace”.

“An irrational decision by the leadership of the Russian Federation will cause significant damages to the Russian state.”

He called for harder sanctions against Russia, declaring that “it’s necessary to isolate a lunatic and not just to defend ourselves by words but also by deeds".

Italy

Italian Prime Minister Mario Draghi demanded Russia “withdraw unconditionally” from Ukraine, saying the invasion of the pro-Western nation “concerns all of us, our lives as free people, our democracy”.

In a televised addressed, he said Italy was “fully aligned” with its partners and would decide “on a very tough package of sanctions against Russia”.

“We have made it clear in every forum that we are ready to impose severe consequences if Russia ... rejects our attempts to resolve the crisis through political means. Now is the time to apply them,” he said.

Rome would “do whatever it takes to preserve Ukraine's sovereignty, Europe's security, and the integrity of the international order based on the rules and values we all share,” he said.

Turkey

Turkish President Recep Tayyip Erdogan denounced Russia's invasion of Ukraine as a “heavy blow” to regional peace.

“We reject Russia's military operation,” Erdogan said in a televised speech, calling it a “heavy blow to regional peace and stability”.

He expressed “sincere sadness” over the fact that the two countries “with which we have close political, economic and social ties” were confronting each other.

Germany

German Chancellor Olaf Scholz said Putin is "bringing suffering and destruction to his direct neighbours, he is violating the sovereignty and borders of Ukraine."

“He is endangering the lives of countless innocent people (and) ... the peace order on our continent. For all that there is no justification. This is Putin's war," he added.

Canada

Canadian Prime Minister Justin Trudeau condemned Russia's “egregious” and “unprovoked” attack on Ukraine, calling on Moscow to immediately withdraw from the country and saying its aggression “will not go unpunished”.

“Canada condemns in the strongest possible terms Russia's egregious attack on Ukraine,” Trudeau said in a statement issued in the hours after Russian President Vladimir Putin announced the start of military operations.

The Canadian leader called the “unprovoked actions” a clear further violation of Ukraine's sovereignty and territorial integrity, and called on Russia to “immediately cease all hostile and provocative actions against Ukraine and withdraw all military and proxy forces from the country.”

Trudeau warned of “severe consequences” for Moscow, vowing with allies “to collectively respond to these reckless and dangerous acts,” including imposing more sanctions.

“Russia's brazen acts will not go unpunished,” he said.