By Maria Tsvetkova and Aleksandar Vasovic

KYIV/MOSCOW: Russian President Vladimir Putin ordered his military command to put nuclear-armed forces on high alert on Sunday as Ukrainian fighters defending the city of Kharkiv said they had repelled an attack by invading Russian troops.

The United States responded by saying Putin was escalating the war in a "totally unacceptable" way.

On the fourth day of the biggest assault on a European state since World War Two, the Ukrainian president's office said negotiations between Kyiv and Moscow would be held at the Belarusian-Ukrainian border. They would meet without preconditions, it said.

As missiles rained down on Ukrainian cities, thousands of Ukrainian civilians, mainly women and children, were fleeing from the Russian assault into neighbouring countries.

The capital Kyiv was still in Ukrainian government hands, with President Volodymyr Zelenskiy rallying his people despite Russian shelling of civilian infrastructure.

But Putin, who has described the invasion as a "special military operation", thrust an alarming new element into play on Sunday when he ordered Russia's deterrence forces - a reference to units which include nuclear arms - onto high alert.

He cited aggressive statements by NATO leaders and economic sanctions imposed by the West against Moscow.

"As you can see, not only do Western countries take unfriendly measures against our country in the economic dimension - I mean the illegal sanctions that everyone knows about very well - but also the top officials of leading NATO countries allow themselves to make aggressive statements with regards to our country," Putin said on state television.

Putin previously referred to his nuclear arsenal in a speech prior to the start of the invasion on Thursday, saying Russia's response to any country that tried to hinder the operation would be immediate and carry "consequences that you have never encountered in your history."

France's Foreign Minister retorted the same day that when making such threats Putin should remember that NATO too was a nuclear alliance.

Ukraine's foreign minister Dmytro Kuleba said Putin's deterrence order was an attempt to pressure Kyiv during talks but it would not be cowed. If Putin used nuclear weapons against Ukraine it would be a catastrophe for the world, he said.

U.S. Ambassador to the United Nations Linda Thomas-Greenfield told CBS: "President Putin is continuing to escalate this war in a manner that is totally unacceptable and we have to continue to stem his actions in the strongest possible way".

In other developments, Russian troops blew up a natural gas pipeline in Kharkiv before daybreak, a Ukrainian state agency said, sending a burning cloud up into the darkness.

Both Ukraine's gas pipeline operator and Kremlin-controlled energy giant Gazprom said the transit of Russian gas via Ukraine, vital for Europe's energy needs, was unaffected.

Ukraine's Western allies ratcheted up their response to Russia's land, sea and air invasion with an almost blanket ban on Russian airlines using European airspace.

And in the strongest economic sanctions yet on Moscow, the United States and Europe said on Saturday they would banish big Russian banks from the main global payments system and announced other measures aimed at limiting Moscow's use of a $630 billion war chest of central bank reserves.

Battle for Khaekiv

Russian soldiers and armoured vehicles rolled into Kharkiv, located in Ukraine's northwest and its second largest city, on Sunday and witnesses reported firing and explosions. But city authorities said Ukrainian fighters had repelled the attack.

"Control over Kharkiv is completely ours! The armed forces, the police, and the defence forces are working, and the city is being completely cleansed of the enemy," regional Governor Oleh Sinegubov said.

Reuters was unable to corroborate the information.

Ukrainian forces were also holding off Russian troops advancing on Kyiv.

"We have withstood and are successfully repelling enemy attacks. The fighting goes on," Zelenskiy said in a video message from the streets of Kyiv.

He has declined to leave the city and has been marshalling combatants and civilians, with many non-combatants seeking shelter in subway stations.

A U.N relief agency said more than 368,000 refugees, have crossed into neighbouring countries, clogging railways, roads and borders.

At least 198 Ukrainians, including three children, have been killed in the invasion, the head of Ukraine's Health Ministry said.

A United Nations agency reported 64 civilian deaths and a Ukrainian presidential adviser said about 3,500 Russian soldiers had been killed or wounded. Reuters was not able to verify the numbers. Western officials have said intelligence showed Russia suffering higher casualties than expected, but Russia has not released casualty figures and Reuters was unable to verify tolls.

No other answer

Germany said it would increase spending on defence to more than 2% of its economic output in response to the invasion, breaking its post-World War Two custom.

"There could be no other answer to Putin's aggression," , Chancellor Olaf Scholz told lawmakers, a day after Germany agreed to send defensive anti-tank weapons, surface-to-air missiles and ammunition to Ukraine.

The United States and its allies authorized more weapons transfers to help Ukraine fight and imposed a range of sanctions on Russia in response to the assault, which threatens to upend Europe's post-Cold War order.

Ignoring weeks of frantic diplomacy and sanctions threats by Western nations seeking to avoid war, Putin has justified the invasion saying "neo-Nazis" rule Ukraine and threaten Russia's security - a charge Kyiv and Western governments say is baseless propaganda.

Putin has said he must eliminate what he calls a serious threat to his country from its smaller neighbour, accusing it of genocide against Russian-speakers in eastern Ukraine - something Kyiv and its Western allies reject as a lie.

Ukraine, a democratic nation of 44 million people, won independence from Moscow in 1991 at the fall of the Soviet Union and has pushed to join the NATO Western military alliance and the EU, goals Russia opposes.

On Saturday, the United States and its allies moved to block certain Russian banks' access to the SWIFT international payment system, making it harder for Russia to trade and for its companies to do business.

They also plan to impose restrictions on Russia's central bank to limit its ability to support the rouble and finance Putin's war effort.

"We will hold Russia to account and collectively ensure that this war is a strategic failure for Putin," the leaders of the European Commission, France, Germany, Italy, Britain, Canada and the United States wrote.