The Russian and Ukrainian governments on Friday signaled an openness to negotiations even as authorities in Kyiv urged citizens to help defend the capital from advancing Russian forces in the worst European security crisis in decades.

Ukraine and Russia will consult in coming hours on a time and place for talks, Ukrainian President Volodymyr Zelenskiy's spokesman Sergii Nykyforov said on social media, offering the first glimmer of hope for diplomacy since the invasion began.

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, but that Ukraine had proposed Warsaw as the venue. That, according to Russian spokesman Dmitry Peskov, resulted in a "pause" in contacts.

"Ukraine was and remains ready to talk about a ceasefire and peace," Nykyforov said in a post on Facebook. "We agreed to the proposal of the President of the Russian Federation."

But U.S. State Department spokesman Ned Price said Russia's offer was an attempt to conduct diplomacy "at the barrel of a gun", and that President Vladimir Putin's military must stop bombing Ukraine if it was serious about negotiations.

The diplomatic overtures stood in stark contrast to events unfolding on the ground and Putin's harsh rhetoric against Ukrainian leaders, including a call for a coup by the country's military.

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

Zelenskiy filmed himself with aides on the streets of the capital, vowing to defend Ukraine's independence.

Some families cowered in shelters after Kyiv was pounded on Thursday night by Russian missiles. Others tried desperately to get on packed trains headed west, some of the hundreds of thousands who have left their homes to find safety, according to the United Nations' aid chief.

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.

The United States imposed sanctions on Putin, Foreign Minister Sergei Lavrov, Defense 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.

However, the steady ramping-up of economic restrictions has not deterred Putin.

Moscow said on Friday 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 Ukrainian authorities reported heavy fighting there.

Early on Saturday, Ukraine's air force command reported heavy fighting near the air base at Vasylkiv southwest of Kyiv, 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.

The mayor of Kyiv and its 3 million people, former world heavyweight boxing champion Vitali Klitschko, said on Friday Russian saboteurs had already entered the city. "The enemy wants to put the capital on its knees and destroy us," he said.

'GLORY TO OUR DEFENDERS'

In New York, Russia vetoed a draft U.N. Security Council resolution that would have deplored Moscow's 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 favor.

Amid the chaos of war, a picture of what was happening on the ground across Ukraine - the second largest country in Europe after Russia itself - 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.

Glory to our defenders, both male and female, glory to Ukraine," he said, flanked by the prime minister and advisors in a video posted to confirm he was in the capital.

Witnesses said they had heard explosions and gunfire near the airport in Kharkiv, Ukraine's second city, close to Russia's border. Ukraine's military said Russian troops had been stopped with heavy losses near the northeastern city of Konotop.

Britain's defence ministry said Russian armoured forces had opened a new route of advance towards the capital after failing to take Chernihiv.

Ukraine said more than 1,000 Russian soldiers had been killed so far. Russia did not release casualty figures.

U.S. 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.

United Nations aid chief Martin Griffiths said hundreds of thousands of people were on the move in Ukraine.

The White House asked Congress for $6.4 billion in security and humanitarian aid for the crisis, officials said.

Ukraine has banned men of fighting age from leaving, and at borders with Poland, Romania, Hungary and Slovakia, those seen crossing by Reuters journalists were mostly women and children.

White House spokeswoman Jen Psaki said the coordinated decision to sanction Putin - something U.S. President Joe Biden had avoided until now - was intended to send a clear message of allied unity.

Russia's foreign ministry said the new sanctions reflected the West's "absolute impotence," the RIA news agency reported.

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.

On Friday, European soccer's governing body moved May's Champions League final from St Petersburg to Paris, and Formula One cancelled this year's Russian Grand Prix. The European Broadcasting Union shut Russia out of the widely watched Eurovision Song Contest for 2022.

'BURN IN HELL'

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

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.

He says he does not plan a military occupation, only to disarm Ukraine and remove its leaders, but it is not clear how a pro-Russian leader could be installed unless Russian troops control much of the country.

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.

As air raid sirens wailed over Kyiv for a second day, some residents sheltered in underground metro stations.

Windows were blasted out of a 10-storey apartment block near the main airport. "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 shelter beneath one building after a televised warning of air strikes.

"How can you wage a war against peaceful people?" said Viktoria, 35, as her children aged 5 and 7 slept in their winter coats.