World leaders on Thursday condemned Russia's full-scale war on Ukraine, vowed to both tighten sanctions and hold Moscow accountable. One leader described the offensive as an "an unjustified barbarian act."

The turmoil from the beginning of a long-feared act of aggression rippled from Europe to Asia. Stock markets plunged, oil prices surged and European aviation officials warned of a high risk to civilian aircraft over Ukraine, reminding air operators that "this is now an active conflict zone."

In New York, the United Nations Security Council held an extraordinary emergency meeting meant to dissuade Russia from sending troops into Ukraine. Secretary-General Antonio Guterres' plea to "give peace a chance" came just as Russian President Vladimir Putin appeared on Russian TV to announce a military operation that he maintained was to protect civilians in Ukraine.

Putin, who said that rebels in eastern Ukraine had asked Moscow for military assistance, warned other countries that any effort to interfere with the Russian operation would lead to "consequences they have never seen."

"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 United States president said shortly after the operation began.

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

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

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," Jens Stoltenberg said in a statement, describing it as a "grave breach of international law, and a serious threat to Euro-Atlantic security."

As leaders across Asia and Europe scrambled to condemn the attack, explosions were heard in Kyiv and other cities in Ukraine. Nations around the world this week have also imposed a raft of new sanctions on Russia.

"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 Council President Charles Michel and European Commission President Ursula von der Leyen said on Twitter. "We will hold the Kremlin accountable," they added.

Moscow had massed more than 150,000 troops on Ukraine’s borders. On Monday, Putin recognized the independence of two separatist regions in eastern Ukraine and ordered Russian forces there for what he called "peacekeeping." Guterres at the U.N. disputed that, saying the troops were entering another country without its consent.

The top European Union chiefs said Thursday that the invasion of Ukraine "will impose massive and severe consequences on Russia" and that there will be more sanctions.

The 27-nation bloc had already imposed some sanctions to punish Moscow for its recognition of breakaway republics in Ukraine earlier this week. EU leaders will discuss additional and harsher punitive measures at a summit meeting later Thursday.

"I am appalled by the horrific events in Ukraine and I have spoken to President (Volodymyr) Zelenskyy to discuss next steps," the British Prime Minister Boris Johnson tweeted.

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

"Russia must immediately put an end to its military operations," French President Emmanuel Macron wrote on Twitter, saying Russia had made the decision to "wage war" on Ukraine.

"France stands in solidarity with Ukraine. It stands by Ukrainians and is working with its partners and allies to end the war," he added.

"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 U.N.," Canadian Prime Minister Justin Trudeau said in a statement.

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," Trudeau added.

German Chancellor Olaf Scholz called Russia’s attack "a terrible day for Ukraine and a dark day for Europe" and "a blatant violation of international law. It cannot be justified by anything."

In a bulletin Thursday, the EU Aviation Safety Agency said "there is a risk of both intentional targeting and misidentification of civil aircraft" and that "the presence and possible use of a wide range of ground and airborne warfare systems poses a HIGH risk for civil flights operating at all altitudes and flight levels."

Asian stock markets were in turmoil.

Market benchmarks in Tokyo and Seoul fell 2% and Hong Kong and Sydney lost more than 3% Thursday. Oil prices jumped nearly $3 per barrel on unease about possible disruption of Russian supplies.

Earlier, Wall Street’s benchmark S&P 500 index fell 1.8% to an eight-month low.

Russia seized Ukraine’s Crimea Peninsula in 2014, and pro-Russia rebels have since been fighting Ukrainian forces in the eastern areas of Donetsk and Luhansk. More than 14,000 people have been killed in the conflict.

Czech Prime Minister Petr Fiala condemned the Russian attack as "absolutely unjustified act of aggression against a sovereign state." Slovakia’s Prime Minister Eduard Heger called it "an unjustified barbarian act."

Many world capitals were trying to determine just what was happening in the tense and fast-changing early moments of a chaotic and violent event.

"We are putting all our effort into collecting the information and comprehending the situation," Japanese Prime Minister Fumio Kishida said shortly after Putin's announcement. "It’s important and challenging to secure the safety of Japanese people (in Ukraine). After fully comprehending the situation, we will handle the case properly," Kishida explained.

Asked whether Taiwan would cooperate with the U.S. and like-minded countries to put export controls on semi-conductors and technology products that the self-ruled island is known for, Foreign Ministry spokesperson Joanne Ou said, "they were in close coordination" and would take the "appropriate actions in response to help Ukraine and uphold the area’s peace and stability."

China, which has denounced sanctions against Russia, advised its citizens in Ukraine to stay home and place a Chinese flag in or on their vehicles if they need to travel long distances.

"Social order is chaotic and out of control, especially in the cities where at times of serious unrest, walking on the streets could make one a target of attack, traffic could be stopped at any time and venturing out creates the possibility of running into uncontrollable risks," the notice said.

China has increasingly aligned its foreign policy with Russia to challenge the West, and has blamed the U.S. and its allies for provoking Moscow.

Australian Prime Minister Scott Morrison said sanctions against Russia would become law Friday but would not take effect until the end of March. He said the time was needed to give "opportunities for businesses that have had very legitimate operations and business interests in Russia and in the affected territories of Ukraine to be able to make changes to their arrangements."

Morrison said that financial sanctions and travel bans that target eight members of the Russian Security Council will be a first batch of measures in response to Russian aggression toward Ukraine. Australia will also align with the U.S. and Britain by targeting two Russian banks.

"The reason we’re doing this is there must be a price for the unprovoked, unlawful, unwarranted, unjustified attacks and threats and intimidation that has been imposed by Russia on Ukraine. This cannot be a consequence-free action by Vladimir Putin and the Russian regime," Morrison said.