Russia-Ukraine: What to know as explosions boom over Ukraine

U.S. President Joe Biden says the world will “hold Russia accountable," and NATO's head called Russia's action a violation of international law and a threat to the security of Europe and its Atlantic allies.

The Ukrainian president earlier rejected Moscow’s claims that his country poses a threat to Russia and made a passionate plea for peace.

Before Putin's announcement, world leaders worked to maintain a united stance and vowed to impose tougher sanctions in the event of a full-fledged invasion.

Putin's declaration came even as the U.N. Security Council was in an emergency meeting Wednesday night on the crisis, at Ukraine's request.

Also read: Russia may be engaged in full-blown invasion of Ukraine before night is over, says Blinken

Here are the things to know about the conflict over Ukraine and the security crisis in Eastern Europe:

Putin said the military operation was needed to protect civilians in eastern Ukraine — a claim the U.S. had predicted he would falsely make to justify an invasion.

In a televised address aired before dawn Thursday Moscow time, Putin accused the U.S. and its allies of ignoring Russia’s demands to prevent Ukraine from joining NATO and offer Moscow security guarantees. He said Russia’s goal was not to occupy Ukraine.

Putin urged Ukrainian servicemen to “immediately put down arms and go home.” In a stark warning to other countries, Putin said: “I have a few words for those who could feel tempted to interfere with ongoing developments. Whoever tries to impede us, let alone create threats for our country and its people, must know that the Russian response will be immediate and lead to the consequences you have never seen in history."

Soon after, the AP heard explosions in the cities of Odesa and Kharkiv.

Leaders of the United States and NATO quickly condemned Russia's attack as unprovoked and unjustified.

Putin “has chosen a premeditated war that will bring a catastrophic loss of life and human suffering,” Biden said in a statement after Putin's announcement.

Biden promised united and decisive responses by the United States and its allies. “The world will hold Russia accountable,” he said.

NATO Secretary-General Jens Stoltenberg called any Russian attack a violation of international law and a threat to the security of Europe and its partners.

"Despite our repeated warnings and tireless efforts to engage in diplomacy, Russia has chosen the path of aggression against a sovereign and independent country,'' the NATO leader said.

Speaking in Russian, President Volodymyr Zelenskyy gave an emotional address early Thursday, before Putin announced the start of the offensive on Zelenskyy's country.

“The people of Ukraine and the government of Ukraine want peace,” he said. “But if we come under attack, if we face an attempt to take away our country, our freedom, our lives and lives of our children, we will defend ourselves. When you attack us, you will see our faces, not our backs.”

Zelenskyy said he asked for a call with Putin late Wednesday but the Kremlin didn't respond.

Earlier Wednesday, Ukraine imposed a nationwide state of emergency, which allows authorities to impose restrictions on movement, block rallies and ban political parties and organizations.

At an emergency meeting of the U.N. Security Council called by Ukraine because of the imminent threat of a Russian invasion, members still unaware of Putin’s announcement of a military operation appealed to him to stop an attack.

U.N. Secretary-General Antonio Guterres opened the meeting, and just before the announcement, he told Putin: “Stop your troops from attacking Ukraine. Give peace a chance. Too many people have already died.”

Guterres later pleaded with Putin, “In the name of humanity, bring your troops back to Russia.”

Ukraine's forces are no match for Moscow's military might, so Kyiv is counting on other countries to hit Russia hard — with sanctions.

Ukrainian Foreign Minister Dmytro Kuleba said on Twitter that the West should target Putin where it hurts without delay. “Hit his economy and cronies. Hit more. Hit hard. Hit now,” Kuleba wrote.

Biden on Wednesday allowed sanctions to move forward against the company that built the Russia-to-Germany Nord Stream 2 gas pipeline and against the company’s CEO.

Biden waived sanctions last year when the project was almost completed, in return for an agreement from Germany to take action against Russia if it used gas as a weapon or attacked Ukraine. Germany said Tuesday it was indefinitely suspending the pipeline.

Ukraine's Western supporters said they had already sent out a strong message with a first batch of sanctions on Tuesday. They said Russian troops moving beyond the separatist-held regions would produce more painful sanctions and possibly the biggest war in a generation on Europe’s mainland.

“This is the toughest sanctions regime we’ve ever put in place against Russia,” British Foreign Secretary Liz Truss said of measures that target key banks that fund the Russian military and oligarchs. "But it will go further, if we see a full-scale invasion of Ukraine.”

The European Union finalized a similar package, which also targets legislators in the lower house of Russia's parliament and makes it tougher for Moscow to get on EU financial and capital markets.

U.S. actions announced Tuesday target high-ranking Russian officials and two Russian banks considered especially close to the Kremlin and Russia’s military, with more than $80 billion in assets.

It is Ukraine, not Russia, where the economy is eroding the fastest under the threat of war.

One by one, embassies and international offices in Kyiv have closed. Flight after flight was canceled when insurance companies balked at covering planes arriving in Ukraine. Hundreds of millions of dollars in investment dried up within weeks.

The squeezing of Ukraine’s economy is a key destabilizing tactic in what the government describes as “hybrid warfare” intended to eat away at the country from within.

The economic woes include restaurants that dare not keep more than a few days of food on hand, stalled plans for a hydrogen production plant that could help wean Europe off Russian gas and uncertain conditions for shipping in the Black Sea, where container ships must carefully edge their way around Russian military vessels.

Ukraine’s parliament and other government and banking websites were hit with another wave of distributed-denial-of-service attacks Wednesday.

Unidentified attackers had also infected hundreds of computers with destructive malware, cybersecurity researchers said.

Officials have long said they expect cyberattacks to precede and accompany any Russian military incursion, and analysts said the incidents hew to a nearly two-decade-old Russian playbook of wedding cyber operations with real-world aggression.

Russian state media are portraying Moscow as coming to the rescue of war-torn areas of eastern Ukraine that are tormented by Ukraine’s aggression.

TV presenters are professing the end of suffering for the residents of the breakaway regions.

“You paid with your blood for these eight years of torment and anticipation,” anchor Olga Skabeyeva said during a popular political talk show Tuesday morning. “Russia will now be defending Donbas.”

Channel One struck a more festive tone, with its correspondent in Donetsk asserting that local residents “say it is the best news over the past years of war.”

“Now they have confidence in the future and that the years-long war will finally come to an end,” she said.

Whether ordinary Russians are buying it is another question.