Russian and Ukrainian forces have clashed on the outskirts of the Ukrainian capital as authorities urged citizens to help defend the city from advancing Russian forces in the worst European security crisis in decades.

Heavy, frequent artillery fire and intense gunfire, apparently some distance from the city centre, could be heard on Saturday in Kyiv in the early hours, a Reuters witness said. 

The Ukrainian military said Russian troops attacked an army base on a main Kyiv avenue but the assault was repelled.

But even as the fighting grew more intense, the Russian and Ukrainian governments signalled an openness to negotiations, offering the first glimmer of hope for diplomacy since Russian President Vladimir Putin launched the invasion on Thursday.

“The fate of Ukraine is being decided right now,” Ukrainian President Volodymyr Zelenskiy said on Friday in a video address posted to his Telegram channel. “Tonight, they will launch an assault. All of us must understand what awaits us. We must withstand this night.”

The air force command reported heavy fighting near the air base at Vasylkiv southwest of the capital, which it said was under attack from Russian paratroopers.

It also said one of its fighters had shot down a Russian transport plane. Reuters could not independently verify the claims.

Kyiv residents were told by the defence ministry to make petrol bombs to repel the invaders, as witnesses reported hearing artillery rounds and intense gunfire from the western part of the city.

After weeks of warnings from Western leaders, Putin unleashed a three-pronged invasion of Ukraine from the north, east and south on Thursday, in an attack that threatened to upend Europe’s post-Cold War order.

“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.”

Putin has cited the need to “denazify” Ukraine’s leadership as one of his main reasons for invasion, accusing it of genocide against Russian-speakers in eastern Ukraine. Kyiv and its Western allies dismiss the accusations as baseless propaganda.

Ukrainians voted overwhelmingly for independence at the fall of the Soviet Union and Kyiv hopes to join NATO and the EU – aspirations that infuriate Moscow.

Putin says Ukraine, a democratic nation of 44 million people, is an illegitimate state carved out of Russia, a view Ukrainians see as aimed at erasing their more than thousand-year history.

Western countries have announced a barrage of sanctions on Russia, including blacklisting its banks and banning technology exports. But they have so far stopped short of forcing it out of the SWIFT system for international bank payments.

The United States imposed sanctions on Putin, Foreign Minister Sergei Lavrov, Defence Minister Sergei Shoigu and Chief of General Staff Valery Gerasimov. The European Union and Britain earlier froze any assets Putin and Lavrov held in their territory. Canada took similar steps.

But amid the chaos of war came a ray of hope.

A spokesman for Zelenskiy said Ukraine and Russia would consult in coming hours on a time and place for talks.

The Kremlin said earlier it offered to meet in the Belarusian capital Minsk after Ukraine expressed a willingness to discuss declaring itself a neutral country while Ukraine had proposed Warsaw as the venue. That, according to Russian spokesman Dmitry Peskov, resulted in a “pause” in contacts.

But US State Department spokesman Ned Price said Russia’s offer was an attempt to conduct diplomacy “at the barrel of a gun” and that Putin’s military must stop bombing Ukraine if it was serious about negotiations.

At the United Nations, Russia vetoed a draft Security Council resolution that would have deplored its invasion, while China abstained, a move Western countries viewed as proof of Russia’s isolation. The United Arab Emirates and India also abstained while the remaining 11 members voted in favour.

A picture of what was happening on the ground across Ukraine – the largest country in Europe after Russia – was slow to emerge.

Zelenskiy wrote on Twitter that there had been heavy fighting with deaths at the entrance to the eastern cities of Chernihiv and Melitopol, as well as at Hostomel.

Ukraine said more than 1000 Russian soldiers had been killed. Russia did not release casualty figures. Zelenskiy said late on Thursday that 137 soldiers and civilians been killed in the fighting, with hundreds wounded.

US Secretary of State Antony Blinken spoke to his Ukrainian counterpart and condemned reported civilian deaths, including those of Ukrainian children, in attacks around Kyiv, the State Department said.

The White House asked Congress for $US6.4 billion ($A8.9 billion) in security and humanitarian aid for the crisis, officials said.

Air raid sirens wailed over Kyiv for a second day on Friday as residents sheltered in underground metro stations.

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

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