As Russian troops attacked Ukraine, world leaders reacted Thursday with outrage — one called it “an unjustified barbarian act” — and vowed to both tighten sanctions and hold the Kremlin accountable.

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 high risk to civilian aircraft over Ukraine, reminding air operators that “this is now an active conflict zone.”

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In New York, the UN 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.”

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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.

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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 EU chiefs said Thursday that the invasion of Ukraine “will impose massive and severe consequences on Russia” and that there will be more sanctions.

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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.