WASHINGTON: World leaders swiftly condemned Russia's military attack on Ukraine on Thursday, with Western capitals pledging new sanctions on Moscow and the UN chief demanding the conflict "stop now."

Here are some reactions to the Russian operation:

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," the US president 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 added.

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.

Biden says 'world will hold Russia accountable' over Ukraine attack

"President Putin has chosen a path of bloodshed and destruction by launching this unprovoked attack on Ukraine. The UK and our allies will respond decisively."

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," wrote EU chiefs Ursula von der Leyen and Charles Michel on Twitter.

"We will hold the Kremlin accountable."

NATO chief Jens Stoltenberg

The NATO head 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 are to hold an emergency meeting on Thursday morning European time to discuss the attack.

Canadian Prime Minister Justin Trudeau

"These unprovoked actions are a clear further violation of Ukraine's sovereignty and territorial integrity, and of Russia's obligations under international law and the Charter of the UN," Trudeau said in a statement.

Germany lashes Russia's 'day of shame,' warns of cost

He said he would meet with partners from the Group of Seven to shape a collective response, "including by imposing sanctions additional to those announced earlier this week."

"These reckless and dangerous acts will not go unpunished."

Polish Prime Minister Mateusz Morawiecki

"We must immediately respond to Russia's criminal aggression on Ukraine Europe and the free world has to stop Putin," Morawiecki wrote on Twitter.

"Today's European Council should approve fiercest possible sanctions. Our support for Ukraine must be real."

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."

"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.

Ukraine's UN ambassador

During the charged UN emergency meeting, the Ukraine's ambassador Sergiy Kyslytsya implored the council, chaired by Russia to "do everything possible to stop the war".

Timeline: Russia's stand-off with Ukraine

He demanded that Russia's ambassador relinquish his duties as chair.

"There is no purgatory for war criminals. They go straight to hell, ambassador," the visibly emotional Kyslytsya said.