Ukrainian forces battled Russian invaders on three sides on Thursday after Moscow mounted an assault by land, sea and air in the biggest attack on a European state since World War Two, prompting tens of thousands of people to flee their homes.

After Russian President Vladimir Putin declared war in a pre-dawn televised address, explosions and gunfire were heard throughout the morning in Kyiv, a city of 3 million people.

“This is a premeditated attack,” U.S. President Joe Biden told reporters at the White House as he unveiled harsh new sanctions coordinated with allies. “Putin is the aggressor. Putin chose this war. And now he and his country will bear the consequences.”

By nightfall, a picture was emerging of fierce fighting across multiple fronts.

An adviser to the Ukrainian presidential office said Russian forces had captured the Chernobyl former nuclear power plant, just 90 km (60 miles) north of the capital, and Hostomel airport in the Kyiv region, where paratroopers had earlier been landed.

Heavy exchanges of fire were also taking place in the regions of Sumy and Kharkiv in the northeast and Kherson and Odessa, home to a populous city and Ukraine’s most important seaport, in the south.

The highway heading west out of Kyiv was choked with traffic across five lanes as residents fled, fearful of bombardments while stuck in their cars.

The U.N. refugee agency said an estimated 100,000 Ukrainians had fled their homes and that several thousand crossed into neighbouring countries, mainly Romania and Moldova.

‘NEW IRON CURTAIN’

The day had begun with missiles raining down on Ukrainian targets and authorities reporting columns of troops pouring across the borders from Russia and Belarus to the north and east, and landing on the southern coasts from the Black Sea and Azov Sea.

The assault brought a calamitous end to weeks of fruitless diplomatic efforts by Western leaders to avert war.

President Volodymyr Zelenskiy called on Ukrainians to defend their country and said arms would be given to anyone prepared to fight.

“What we have heard today are not just missile blasts, fighting and the rumble of aircraft. This is the sound of a new Iron Curtain, which has come down and is closing Russia off from the civilised world,” Zelenskiy said.

In his address, Putin said he had ordered “a special military operation” to protect people, including Russian citizens, subjected to “genocide” in Ukraine – an accusation the West calls baseless propaganda.

“And for this we will strive for the demilitarisation and denazification of Ukraine,” Putin said.

After referring earlier in his speech to Russia’s powerful nuclear arsenal, he also warned: “Whoever tries to hinder us … should know that Russia’s response will be immediate. And it will lead you to such consequences that you have never encountered in your history.”

France’s Foreign Minister Jean-Yves Le Drian later said Putin should understand that NATO was also a nuclear alliance.

Biden has ruled out sending U.S. troops to defend Ukraine, but Washington has reinforced its NATO allies in the region with extra troops and planes.

After consulting counterparts from the Group of Seven leading industrialised nations, Biden announced measures to impede Russia’s ability to do business in the world’s major currencies, along with sanctions against banks and state-owned enterprises.

Britain also targeted banks, along with members of Putin’s closest circle and super-rich Russians who enjoy high-rolling London lifestyles.

European Union leaders said measures would include freezing Russian assets in the 27-nation bloc, halting banks’ access to European financial markets and hitting “Kremlin interests”.

Canada and Switzerland were among a number of other countries to announce new measures targeting Russia. China remained out of step, however, rejecting the description of Russia’s actions as an “invasion”.

Russia is one of the world’s biggest energy producers, and both it and Ukraine are among the top exporters of grain. War and sanctions will disrupt economies around the world already facing a supply crisis as they emerge from the coronavirus pandemic.

Stocks pared losses and the U.S. dollar and oil futures pared gains after Biden’s sanctions announcement. Brent oil earlier surged past $100/barrel for the first time since 2014.

‘HANG THEM FROM BRIDGES’

Even with a full-blown invasion under way, Putin’s ultimate aim is obscure. He said he did not plan a military occupation, only to disarm Ukraine and purge it of nationalists.

The outright annexation of such a vast, hostile country could be beyond even Russia’s military capabilities.

A senior U.S. defence official said Washington believed the invasion was intended to “decapitate” Zelenskiy’s government.

But it is hard to see Ukrainians accepting leadership installed by Moscow.

“I think we must fight all those who invade our country so strongly,” said one man stuck in traffic trying to leave Kyiv. “I would hang every single one of them from bridges.”

A democratic nation of 44 million people, Ukraine is Europe’s biggest country by area after Russia itself. It voted for independence at the fall of the Soviet Union and has recently stepped up efforts to join NATO and the European Union, aspirations that infuriate Moscow.

Putin, who denied for months that he was planning an invasion, has called Ukraine an artificial construct carved from Russia by its enemies – a characterisation Ukrainians see as an attempt to erase their more than 1,000-year-old history.

While many Ukrainians, particularly in the east, speak Russian as a native language, virtually all identify themselves as Ukrainian.

FIGHT OR FLEE

A resident of Kharkiv, Ukraine’s second biggest city and close to the Russian border, said windows in apartment blocks were shaking from constant blasts.

Blasts could be heard in the southeastern port of Mariupol, near a frontline held by Russian-backed separatists in eastern Ukraine. Local authorities said 26 people were being treated for wounds in hospital after a district of the port was shelled and “an attempt by Russian troops to break through” into the city was thwarted.

Civilians in Mariupol packed bags. “We are going into hiding,” a woman said.

Ukrainian forces have downed two Russian helicopters and seven other Russian aircraft and destroyed several Russian trucks, and a platoon from Russia’s 74th Motor Rifles Brigade has surrendered, Ukraine’s ambassador to the United States Oksana Markarova said.

Markarova cited earlier figures that Russian attacks had killed 40 Ukrainian servicemen and dozens of civilians.

Russia’s defence ministry said it had destroyed 83 land-based Ukrainian targets and had achieved all its goals, according to Interfax news agency.

In Kyiv, queues of people waited to withdraw money and buy supplies of food and water. Cars stretched for dozens of kilometres (miles) on the highway leading west towards Poland, where Western countries have prepared to receive hundreds of thousands of refugees.

“We’re afraid of bombardments,” said Oxana, stuck in her car with her three-year-old daughter on the backseat. “This is so scary.”