After weeks of warnings from the US and its allies, Putin unleashed a three-pronged invasion of Ukraine from the north, east and south before dawn on Thursday, in the biggest attack on a European state since World War II.

"I once again appeal to the military personnel of the armed forces of Ukraine: do not allow neo-Nazis and (Ukrainian radical nationalists) to use your children, wives and elders as human shields," Putin said at a televised meeting with Russia's Security Council on Friday.

"Take power into your own hands, it will be easier for us to reach agreement."

Putin says he does not plan a military occupation, only to disarm Ukraine and remove its leaders, alluding to Ukrainian nationalists who collaborated with Nazi invaders in World War II to fight Soviet Russia.

But it is not clear how a pro-Russian leader could be installed unless Russian troops control much of the country.

Russia said it had captured the Hostomel airfield northwest of the capital - a potential staging post for an assault on Kyiv that has been fought over since Russian paratroopers landed there in the first hours of the war.

This could not be confirmed and the Ukrainian authorities reported heavy fighting there.

"Shots and explosions are ringing out in some neighbourhoods. Saboteurs have already entered Kyiv," said the mayor of the city of three million, former world heavyweight boxing champion Vitali Klitschko.

"The enemy wants to put the capital on its knees and destroy us."

Amid the chaos of war, a picture of what was happening on the ground was slow to emerge.

President Volodymyr Zelenskiy tweeted that there had been heavy fighting with people killed at the entrance to the eastern cities of Chernihiv and Melitopol as well as at Hostomel.

Witnesses said loud explosions and gunfire could also be heard near the airport in Kharkiv, Ukraine's second city, close to Russia's border, and air raid sirens sounded over Lviv in the west.

Authorities reported heavy fighting in the eastern city of Sumy.

The UK defence ministry said in an intelligence update that Russian armoured forces had opened a new route of advance towards the capital after failing to take Chernihiv, and most troops remained more than 50km from Kyiv's city centre.

Ukraine said more than 1000 Russian soldiers had been killed so far.

Russia did not release casualty figures.

The United Nations said 25 civilians had been killed and 102 wounded, figures that were likely to be a "significant under-estimate".

None of the tolls could be independently verified.

Air raid sirens wailed over Kyiv for a second day and some residents sheltered in underground metro stations.

Windows were blasted out of a 10-storey apartment block near the main airport.

A two-metre crater showed where a shell had struck before dawn.

"How can we be living through this in our time? Putin should burn in hell along with his whole family," said Oxana Gulenko, sweeping broken glass from her room.

Hundreds crowded into a cramped bomb shelter beneath one building after a televised warning of air strikes.

Ukrainians were circulating an unverified recording on Friday of a Russian warship ordering a Ukrainian outpost on the tiny Black Sea island of Zmiinyi (Snake Island) to surrender.

The Ukrainians reply: "Russian warship, go f*** yourself."

Zelenskiy said the 13 guards had been killed by a Russian strike and would receive posthumous honours.

The Russian defence ministry claimed they had taken prisoner 82 Ukrainian troops on the island.

Envoys of the EU's 27 member states agreed to freeze any assets in the EU belonging to Russian President Vladimir Putin and Foreign Minister Sergey Lavrov, adding to a slew of sanctions backed by the leaders at an emergency summit on Thursday night.

Numerous countries have announced new sanctions on Russia, including blacklisting its banks and banning technology imports.

But they have so far stopped short of forcing it out of the SWIFT system for international bank payments, drawing criticism from Ukraine which says there is no reason to hold back.

US officials believe Russia's initial aim is to "decapitate" Zelenskiy's government.

Zelenskiy said he knew he was "the number one target" but would stay in Kyiv.

He shared a brief video of himself along with other senior Ukrainian politicians including Prime Minister Denys Shmygal in a Facebook post on Friday evening, in which the presidential office could be seen in the background.

"We are all here," he said. "We are in Kyiv. We are defending Ukraine."

Zelenskiy's video came in response to rumours that he was hiding in a bunker or had left the city.

An adviser to Zelenskiy said Ukraine was prepared for talks with Russia, including on staying neutral, one of Putin's pre-war demands.

The Kremlin said it had offered talks in the Belarusian capital Minsk but that Ukraine had proposed Warsaw instead and there was now a "pause" in contacts.

Ukrainians were fleeing into neighbouring Poland, Romania, Hungary and Slovakia, mostly women and children after authorities in Kyiv restricted passage for men between 18 and 60 years old.

with reporting from DPA

Australian Associated Press