WARSAW, Poland (AP) — Russian troops launched a broad, three-pronged assault on Ukraine that brought explosions and set off air raid sirens to the country's capital, Kyiv, and other cities, shattering any remaining hope that a military conflict would be avoided.

Ukraine's leadership said at least 40 people had been killed in what it called a "full-scale war" targeting the country from the east, north and south. It said Russia's intent was to destroy the state of Ukraine, a Western-looking democracy intent on escaping Moscow's orbit.

As Ukrainian forces fought back and civilians piled into trains and cars to flee, NATO and European leaders rushed to respond, if not directly in Ukraine, with strong financial sanctions against Russia and moves to strengthen their own borders.

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

PUTIN MAKES HIS MOVE

In a televised address as the attack began, Russian President Vladimir Putin said it was needed to protect civilians in eastern Ukraine, where Ukrainian forces and Russia-backed separatists have been fighting for almost eight years.

The U.S. had predicted Putin would falsely claim that the rebel-held regions were under attack to justify an invasion.

The Russian leader warned other countries that any attempt to interfere in Ukraine would "lead to consequences you have never seen in history" — a dark threat implying Russia was prepared to use its nuclear weapons.

Putin accused the U.S. and its allies of ignoring Russia's demands to block Ukraine from ever joining NATO and offer Moscow security guarantees.

Putin said Russia does not intend to occupy Ukraine but plans to "demilitarize" it. He urged Ukrainian servicemen to "immediately put down arms and go home." Soon after his address, explosions were heard in the cities of Kyiv, Kharkiv and Odesa.

Russia's Defense Ministry reported hours later that the Russian military has destroyed 74 Ukrainian military facilities, including 11 air bases.

UKRAINE'S PRESIDENT URGES CALM

Kyiv residents could be heard shouting in the streets when the first explosions sounded.

Ukrainian President Volodymyr Zelenskyy, who had repeatedly appealed to Putin in recent days to pursue a diplomatic path instead of taking military action, issued a video statement declaring martial law.

He told Ukrainians that the United States was gathering international support to respond to Russia. He urged residents to remain calm and to stay at home, and urged world leaders Thursday to provide defense assistance and help protect Ukraine's airspace.

Later Thursday, Zelenskyy reported that Russian forces were trying to seize the Chernobyl nuclear plant.

The plant was the site of the world's worst nuclear accident in April 1986, when a nuclear reactor exploded and spewed radioactive waste across Europe. The plant lies 130 kilometers (80 miles) north of the capital of Kyiv.

"Our defenders are giving their lives so that the tragedy of 1986 will not be repeated," Zelenskyy said on Twitter.

THE WEST REACTS

World leaders decried the start of an invasion that could cause massive casualties, topple Ukraine's democratically elected government and threaten the post-Cold War balance.

NATO Secretary General Jens Stoltenberg called Russia's attack "a brutal act of war" and said Moscow had shattered peace on the European continent.

U.S. President Joe Biden said Putin "has chosen a premeditated war that will bring a catastrophic loss of life and human suffering."

The leaders of the Group of Seven have strongly condemned "the large-scale military aggression by the Russian Federation against the territorial integrity, sovereignty and independence of Ukraine."

They called "on all partners and members of the international community to condemn this attack in the strongest possible terms, to stand shoulder to shoulder with Ukraine, and raise their voice against this blatant violation of the fundamental principles of international peace and security."

The head of the U.N. refugee agency called on neighboring countries to keep their borders open for Ukrainians fleeing the fighting.

U.N. High Commissioner for Refugees Filippo Grandi pointed to "reports of casualties and people starting to flee their homes to seek safety."

He said his agency had stepped up its operations and capacity in both Ukraine and its neighbors.

WORLD MARKETS FALL

World stock markets plunged and oil prices soared amid concerns that heating bills and food prices would skyrocket.

Beyond its human toll, the conflict looks set to send prices spiraling even higher at gasoline pumps and grocery stores around the world.

Russia and Ukraine are major producers not only of energy products but also grains and various other commodities. War could upend global supplies, as could sanctions brought by the United States and other allies.

WHEN WILL THE WEST IMPOSE MORE SANCTIONS?

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

President Joe Biden is expected to announce on Thursday at least some of the toughest sanctions and financial penalties that the United States can muster in response.

Biden, for weeks, promised "swift and severe costs" if Russian forces moved into Ukraine, and he made clear that the world's biggest economy would go after Russia financially, not militarily.

Biden administration officials described measures that would send Russia's ruble crashing, isolate Moscow from the world financial system and potentially drive the country into recession.

Meanwhile, the European Union planned the "strongest, the harshest package" ever, to be considered at a summit on Thursday, according to EU foreign policy chief Josep Borrell.

"A major nuclear power has attacked a neighbor country and is threatening reprisals of any other states that may come to the rescue," Borrell said.

"This is not only the greatest violation of international law, it's a violation of the basic principles of human co-existence. It's costing many lives with unknown consequences ahead of us. The European Union will respond in the strongest possible terms."

EU BEEFS UP PEACEKEEPING FORCE IN BOSNIA

The European Union-led peacekeeping force in Bosnia announced that it would double the number of its ground troops to prevent ripple effects from the conflict in Ukraine.

The force, known as EUFOR, described the step as a precautionary move. "The deterioration of the security situation internationally has the potential to spread instability" to the ethnically divided Balkan country, it said.

A staunchly pro-Russian Bosnian Serb leader, Milorad Dodik, has for years advocated the separation of the semi-autonomous Bosnian Serb mini-state from the rest of the country.

Last winter, with tacit support from Moscow, Dodik intensified his secessionist campaign, pledging to form an exclusively Serb army, judiciary and tax system.

The EU force announced that four companies of its reserve forces from Austria, Bulgaria, Romania and Slovakia, would deploy to Bosnia over the next two weeks to reinforce its 600-strong contingent already stationed in the country. The new deployments will total 500 troops.

NATO'S EASTERN FLANK

The countries on NATO's eastern flank, all under Soviet domination during the Cold War, are especially nervous.

The Baltic states  of Lithuania, Latvia and Estonia wondered if they could be the Kremlin's next target. Poles were also shaken.

The parliament in Poland, which borders both Ukraine and Belarus, strongly denounced Russia's attack on Ukraine and vowed its support to Ukraine.

Members of the Sejm, or lower house of parliament, approved by acclamation a statement condemning Russia. Polish Prime Minister Mateusz Morawiecki told the assembly that Thursday would go down in history as "the day Russia chose war," attacking another nation for no reason.

U.S. Ambassador Mark Brzezinski sought to assure Poland that it is safe. He noted that there are now 10,000 U.S. soldiers in Poland. More than half were deployed in recent weeks amid the Russian threats.

CHINA'S SUPPORT FOR RUSSIA

China's customs agency on Thursday approved imports of wheat from all regions of Russia, a move that could help to reduce the impact of possible Western sanctions.

China's populous market is a growth area for other farm goods suppliers, but Beijing had barred imports until now from Russia's main wheat-growing areas due to concern about possible fungus and other contamination.

Russia is one of the biggest wheat producers, but its exports would be vulnerable if its foreign markets block shipments in response to its attack on Ukraine.

Thursday's announcement said Russia would "take all measures" to prevent contamination by wheat smut fungus and would suspend exports to China if it was found.

UKRAINE SEES MORE CYBERATTACKS

The websites of Ukraine's defense, foreign and interior ministries were unreachable or painfully slow to load Thursday morning after a punishing wave of distributed-denial-of-service attacks  as Russia struck at its neighbor.

In addition to DDoS attacks on Wednesday, cybersecurity researchers said unidentified attackers had infected hundreds of computers with destructive malware, some in neighboring Latvia and Lithuania.

Officials had long expected cyberattacks to precede and accompany any Russian military incursion.

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Associated Press writers around the world contributed to this report.

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Follow AP's coverage of the tensions between Russia and Ukraine at https://apnews.com/hub/russia-ukraine