MOSCOW — Russian President Vladimir Putin on Thursday announced a military operation in Ukraine and warned other countries that any attempt to interfere with the Russian action would lead to "consequences they have never seen."

He said the attack 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, Putin accused the U.S. and its allies of ignoring Russia's demand to prevent Ukraine from joining NATO and offer Moscow security guarantees. He said Russia's goal was not to occupy Ukraine.

U.S. President Joe Biden denounced the "unprovoked and unjustified" attack on Ukraine and said the world will "hold Russia accountable."

As Putin spoke, big explosions were heard in Kyiv, Kharkiv and other areas of Ukraine.

A full-blown Russian invasion could cause massive casualties and topple Ukraine's democratically elected government. And the consequences of the conflict and resulting sanctions levied on Russia could reverberate throughout the world, affecting energy supplies in Europe, jolting global financial markets and threatening the post-Cold War balance on the continent.

He said the Russian military operation aims to ensure a "demilitarization" of Ukraine. Putin urged Ukrainian servicemen to "immediately put down arms and go home."

Putin announced the military operation after the Kremlin said rebels in eastern Ukraine asked Russia for military assistance to help fend off Ukrainian "aggression." The announcement immediately fueled fears that Moscow was offering up a pretext for war, just as the West had warned.

A short time later, the Ukrainian president rejected Moscow's claims that his country poses a threat to Russia and said a Russian invasion would cost tens of thousands of lives.

"The people of Ukraine and the government of Ukraine want peace," President Volodymyr Zelenskyy said in an emotional overnight address, speaking in Russian in a direct appeal to Russian citizens. "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 to arrange a call with Putin late Wednesday, but the Kremlin did not respond.

In an apparent reference to Putin's move to authorize the deployment of the Russian military to "maintain peace" in eastern Ukraine, Zelensky warned that "this step could mark the start of a big war on the European continent."

"Any provocation, any spark could trigger a blaze that will destroy everything," he said.

He challenged the Russian propaganda claims, saying that "you are told that this blaze will bring freedom to the people of Ukraine, but the Ukrainian people are free."

The United Nations Security Council quickly scheduled an emergency meeting Wednesday night at Ukraine's request. Ukrainian Foreign Minister Dmytro Kuleba called the separatists' request "a further escalation of the security situation."

Anxiety about an imminent Russian offensive against its neighbor soared after Putin recognized the separatist regions' independence on Monday, endorsed the deployment of troops to the rebel territories and received parliamentary approval to use military force outside the country. The West responded with sanctions.

Kremlin spokesman Dmitry Peskov said the rebel chiefs wrote to Putin on Wednesday, pleading with him to intervene after Ukrainian shelling caused civilian deaths and crippled vital infrastructure.

White House press secretary Jen Psaki said the separatists' request for Russian help was an example of the sort of "false-flag" operation that the U.S. and its allies have expected Moscow to use as a pretense for war.

"So we'll continue to call out what we see as false-flag operations or efforts to spread misinformation about what the actual status is on the ground," she said.

Earlier in the day, Ukrainian lawmakers approved a decree that imposes a nationwide state of emergency for 30 days starting Thursday. The measure allows authorities to declare curfews and other restrictions on movement, block rallies and ban political parties and organizations "in the interests of national security and public order."

The action reflected increasing concern among Ukrainian authorities after weeks of trying to project calm. The Foreign Ministry advised against travel to Russia and recommended that any Ukrainians who are there leave immediately.

"For a long time, we refrained from declaring a state of emergency ... but today the situation has become more complicated," Ukrainian National Security and Defense Council head Oleksiy Danilov told parliament, emphasizing that Moscow's efforts to destabilize Ukraine represented the main threat.

Pentagon press secretary John Kirby said the Russian force of more than 150,000 troops arrayed along Ukraine's borders is in an advanced state of readiness. "They are ready to go right now," Kirby said.

The latest images released by the Maxar satellite image company showed Russian troops and military equipment deployed within 10 miles of the Ukrainian border and less than 50 miles from Ukraine's second-largest city, Kharkiv.

Early Thursday, airspace over all of Ukraine was shut down to civilian air traffic, according to a notice to airmen. A commercial flight tracking website showed that an Israeli El Al Boeing 787 flying from Tel Aviv to Toronto turned abruptly out of Ukrainian airspace before detouring over Romania, Hungary, Slovakia and Poland. The only other aircraft tracked over Ukraine was a U.S. RQ-4B Global Hawk unmanned surveillance plane, which began flying westward early Thursday after Russia put in place flight restrictions over Ukrainian territory.

Another wave of distributed-denial-of-service attacks hit Ukraine's parliament and other government and banking websites on Wednesday, and cybersecurity researchers said unidentified attackers had also infected hundreds of computers with destructive malware.

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.

In other developments, Russia evacuated its embassy in Kyiv; Ukraine recalled its ambassador to Russia and considered breaking all diplomatic ties with Moscow and dozens of nations further squeezed Russian oligarchs and banks out of international markets.

President Joe Biden 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.

"As I have made clear, we will not hesitate to take further steps if Russia continues to escalate," Biden said in a statement.

Germany said Tuesday that it was indefinitely suspending the project, after Biden charged that Putin had launched "the beginning of a Russian invasion of Ukraine" by sending troops into the separatist regions. The pipeline is complete but has not yet begun operating.

Putin said Tuesday that he had not yet sent any Russian troops into the rebel regions, contrary to Western claims, and Donetsk rebel leader Denis Pushilin insisted Wednesday there were no Russian troops in the region, even though a local council member claimed the previous day they had moved in.

Already, the threat of war has shredded Ukraine's economy and raised the specter of massive casualties, energy shortages across Europe and global economic chaos.

European Union sanctions against Russia took effect, targeting several companies along with 351 Russian lawmakers who voted for a motion urging Putin to recognize the rebel regions and 27 senior government officials, business executives and top military officers.

The Russian Foreign Ministry has shrugged off the sanctions, saying that "Russia has proven that, with all the costs of the sanctions, it is able to minimize the damage."

In Ukraine's east, one Ukrainian soldier was killed and six more wounded after rebel shelling, the Ukrainian military said Wednesday. Separatist officials reported several explosions on their territory overnight and three civilian deaths.

Facing a barrage of criticism at the 193-member United Nations General Assembly, Russia's U.N. ambassador, Vassily Nebenzia, warned Ukraine that Russia will monitor a cease-fire in the east and emphasized that "no one intends to go softly, softly with any violators."

"A new military adventure" by Kyiv "might cost the whole of Ukraine very dearly," he warned ominously.

After weeks of rising tensions, Putin's steps this week dramatically raised the stakes. He recognized the independence of the separatist regions, a move he said extends even to the large parts of the territories now held by Ukrainian forces, and had parliament grant him authority to use military force outside the country.

Putin laid out three conditions that he said could end the standoff, urging Kyiv to renounce its bid to join NATO, to partially demilitarize and to recognize Russia's sovereignty over Crimea, the Black Sea peninsula that Moscow annexed from Ukraine in 2014. Ukraine long has rejected such demands.

Litvinova reported from Moscow. Angela Charlton in Paris; Frank Jordans in Berlin; Lorne Cook in Brussels, Vladimir Isachenkov in Moscow, Frank Bajak in Boston, Robert Burns, Matthew Lee, Aamer Madhani, Eric Tucker, Ellen Knickmeyer, Zeke Miller, Chris Megerian and Darlene Superville in Washington contributed.

Follow AP's coverage of the Ukraine crisis at https://apnews.com/hub/russia-ukraine