After nightfall, a picture was emerging of fierce fighting across multiple fronts. President Volodymyr Zelenskiy late on Thursday ordered a general mobilization, to be carried out within 90 days, "to ensure the defence of the state".

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 Kyiv. The plant is along the shortest route from the Ukrainian capital to Belarus, where Moscow has staged troops.

There was also fighting at Hostomel airport, just outside Kyiv, where Russian paratroopers landed. A Ukrainian official later said the airfield had been recaptured, while a senior U.S. defence official said Russian forces were advancing closer to Kyiv.

Heavy exchanges of fire were also reported in the regions of Sumy and Kharkiv in the northeast and Kherson in the south.

The highway heading west out of Kyiv, home to 3 million people, was choked with traffic across five lanes as residents sought to escape, fearful of bombardments while stuck in their cars.

The U.N. refugee agency said an estimated 100,000 Ukrainians had fled their homes. Thousands were crossing into neighbouring countries, including Romania, Moldova, Poland and Hungary. read more

Some 57 people were killed and 169 were wounded on Thursday, Ukraine's health minister said, while the interior ministry said 13 border guards died when a Russian vessel shelled Ukraine's Zmiinyi Island, south of the Black Sea port of Odessa.

The day began with missiles raining down on targets across Ukraine and reports of troops and armour pouring across the borders from Russia and Belarus to the north and east.

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.

Putin, after referring earlier in his speech to Russia's powerful nuclear arsenal, 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."

Asked whether that threat was tantamount to threatening Russian use of nuclear weapons, Foreign Minister Jean-Yves Le Drian said it was indeed understood as such, adding that Putin should also understand that NATO was 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.