Leaders of Western nations said they would expand sanctions against Russia ahead of an emergency meeting of G7 leaders.
United States President Joe Biden said he would be co-ordinating with G7 leaders to impose a more extensive set of sanctions against Russia as he criticised Russian President Vladimir Putin's decision to start a "military operation" in Ukraine.
"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," Mr Biden said in a statement.
"Russia alone is responsible for the death and destruction this attack will bring, and the United States and its Allies and partners will respond in a united and decisive way. The world will hold Russia accountable."
He earlier had a phone call with Ukranian President Volodymyr Zelenskyy, saying he would be pushing for action at the United Nations Security Council.
"He asked me to call on the leaders of the world to speak out clearly against President Putin’s flagrant aggression and to stand with the people of Ukraine."
He said he would be meeting with his G7 counterparts within the next 24 hours to outline more extensive measures that will be taken against Russia "for this needless act of aggression against Ukraine and global peace and security".
European Union Foreign Policy Chief Josep Borrell said on Thursday "these are among the darkest hours for Europe since the end of World War II".
"A major nuclear power is attacking a neighbour country and is threatening reprisals of any other state that may come to its rescue," Mr Borrell said.
"This is not only the greatest violation of international law, it's a violation of the basic principles of human coexistence. It's costing many lives with unknown consequences ahead of us. The European Union will respond in the strongest possible terms."
Mr Borrell said a meeting of the European Council has been called to impose on Russia "the harshest package of sanctions we have ever implemented".
Britain's Prime Minister Boris Johnson. Source: AAP
"I am appalled by the horrific events in Ukraine and I have spoken to President Zelenskyy to discuss next steps," he posted on Twitter.
"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."
French President Emmanuel Macron has condemned Russia's military actions in Ukraine and called on Moscow to end them immediately.
"France strongly condemns the decision of Russia to start a war with Ukraine. Russia must immediately put an end to its military operations," wrote Mr Macron on his Twitter account.
France's President Emmanuel Macron. Source: AAP
Estonian Prime Minister Kaja Kallas also took to social media to condemn Russia.
"I condemn in the gravest manner #Russia's large-scale military attack against #Ukraine. Aggression is a crime that demands the clearest international reaction and strong response. We are with Ukraine and the Ukrainian people at this dark hour. We #StandWithUkraine."
German Chancellor Olaf Scholz described the Russian military operation in Ukraine as "a glaring violation" of international law on a "dark day" for Europe.
"Germany condemns in the strongest possible terms this unscrupulous act by President Putin. Our solidarity goes out to Ukraine and its people," he added in a statement.
He urged Moscow to stop its military operation "immediately".
Prime Minister of Finland Sanna Marin also said she strongly condemns the action.
"I strongly condemn the military action Russia has taken in Ukraine. The attack is a grave breach of international law and threatens the life of numerous civilians."
UN Secretary-General António Guterres made a direct, impassioned plea on Thursday to Vladimir Putin to stop - "in the name of humanity" - the Russian military assault on Ukraine.
Speaking after an emergency Security Council session, which coincided with the Russian president's announcement of military operations against Ukraine, a clearly emotional Mr Guterres said it was "the saddest day" of his tenure as UN chief.
"President Putin, in the name of humanity, bring your troops back to Russia. 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," he added.
Japanese Prime Minister Fumio Kishida said Japan strongly condemns the one-sided actions of Russia, following reports that Russian forces had fired missiles at several Ukrainian cities.
Mr Kishida said he instructed the relevant Japanese authorities to do everything possible to ensure the safety of Japanese nationals in Ukraine.
