KYIV: Russian forces reached the outskirts of Kyiv on Friday as Ukrainian President Volodymyr Zelensky said the invading troops were targeting civilians and explosions could be heard in the besieged capital.

Pre-dawn blasts in Kyiv set off a second day of violence after Russian President Vladimir Putin defied Western warnings to unleash a full-scale ground invasion and air assault on Thursday that quickly claimed dozens of lives and displaced at least 100,000 people.

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The United States and its allies responded with a barrage of sanctions, but the Russian forces looked to press home their advantage after a string of key strategic victories on day one.

"Horrific Russian rocket strikes on Kyiv," Ukraine Foreign Minister Dmytro Kuleba posted on Twitter after the initial explosions were heard in the capital on Friday morning.

Loud explosions heard in central Kyiv: AFP

"Last time our capital experienced anything like this was in 1941 when it was attacked by Nazi Germany. Ukraine defeated that evil and will defeat this one."

Zelensky then delivered a video message in which he railed against the Russian troops.

"They say that civilian objects are not a target for them. But this is another lie of theirs. In reality, they do not distinguish between areas in which they operate," Zelensky said.

"Ukrainian air defence systems are defending our skies," he said. "Ukrainians are demonstrating heroism".

At least 137 "heroes" were killed during the first day of fighting, Zelensky had said on Thursday, as he called up conscripts and reservists nationwide.

Zelensky said there was now a "new iron curtain" between Russia and the rest of the world, adding in a later speech his nation had been "left alone".

"Who is ready to fight alongside us? I don't see anyone."

And while the United States moved to impose sanctions on Russian elites and banks, it stressed that American forces would not fight in Ukraine.

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NATO said it had activated "defence plans" for allied countries.But NATO chief Jens Stoltenberg said there was no plan to send alliance forces into Ukraine.

'War on Europe'

Among the highest-profile strategic developments on Thursday, Ukraine said Russian forces had seized the Chernobyl nuclear power plant -- prompting concern from international nuclear watchdogs.

Zelensky called the attack on Chernobyl "a declaration of war on all of Europe" while 18 people were killed at a military base near the Black Sea port of Odessa in the deadliest single strike reported by Kyiv.

Witnesses also told AFP that Russian paratroopers wrested control of the strategic Gostomel airfield, on the northwestern outskirts of Kyiv.

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"The helicopters came in and then the battles started. They were firing machine guns, grenade launchers," resident Sergiy Storozhuk said.

Russian ground forces had initially moved into Ukraine from the north, south and east, forcing many Ukrainians to flee their homes as the sound of bombing reverberated.

Moscow's defence ministry said Thursday its forces had "successfully completed" their objectives for the day, earlier claiming to have destroyed over 70 Ukrainian military targets, including 11 airfields.

Western intelligence confirmed Moscow had established "complete air superiority" over Ukraine.

Ukrainian forces said they had killed "around 50 Russian occupiers" while repulsing an attack on a town on the frontline with Moscow-backed rebels, which could not immediately be confirmed by AFP.

Ukraine also said a military plane with 14 people on board crashed south of Kyiv with officials still determining how many people died, while a transport plane crashed in Russia killing the crew.

Olena Kurilo was among 20 people wounded by flying shards of glass following a blast in the eastern Ukrainian town of Chuguiv.

"Never, under any conditions will I submit to Putin. It is better to die," the 52-year-old teacher said, her face covered in bandages.

In the Ukrainian village of Starognativka near the frontline where separatists have faced off against Kyiv's forces, official Vladimir Vesyelkin said missiles had rained down since Friday morning and power was out.

"They are trying to wipe the village off the face of the earth," he said.

Sanctions

Weeks of diplomacy failed to deter Putin, who massed over 150,000 troops on Ukraine's borders in what the West said was Europe's biggest military build-up since World War II.

Western allies had initially imposed some sanctions on Russia in an effort to deter Putin, then followed through on Thursday with vows to heavily punish Russia economically.

US President Joe Biden announced export controls against Russia to cut off more than half of the country's high-tech imports, alongside sanctions on Russian elites he called "corrupt billionaires", and banks.

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He will meet Friday with fellow Nato leaders in an extraordinary virtual summit to discuss the security situation in and around Ukraine later.

The EU moved to impose "massive" sanctions on Russia's energy and finance sectors, while French President Emmanuel Macron called Putin to "demand immediate halt" to the offensive -- but said the door must be kept open for dialogue.

The fighting spooked global financial markets, with stocks plunging and oil prices soaring past $100.

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IMF chief Kristalina Georgieva said the unrest carried "significant economic risk" for the world, but Putin insisted he did not seek to undermine the global economic system.

Demonstrations across Europe

In a televised address, the Russian president justified the assault as a defence of the self-proclaimed Donetsk and Lugansk republics in eastern Ukraine.

The Kremlin earlier said the leaders of the two separatist territories had asked Moscow for military help against Kyiv after Putin recognised their independence on Monday.

A conflict between the separatists and government forces has dragged on since 2014, killing more than 14,000 people on both sides.

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Russia has also long demanded that Ukraine be forbidden from ever joining NATO and that US troops pull out from Eastern Europe.

Demonstrators took to the streets of European capitals to condemn the Kremlin but a small anti-war protest in Moscow was quickly halted by police, with over 1,700 people detained at rallies across Russia.