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Russian forces have attacked oil and gas facilities in Ukraine, sparking huge explosions, as Western allies prepared new sanctions, including banishing key Russia banks from the main global payments system.

Ukrainian forces were holding off Russian troops advancing on the capital, Kyiv, said President Volodymyr Zelenskiy on Sunday as the biggest assault on a European state since World War Two entered its fourth day.

Russian missiles found their mark, including a strike that set an oil terminal ablaze in Vasylkiv, southwest of Kyiv, the town's mayor said. Blasts sent huge flames and billowing black smoke into the night sky, online posts showed.

There were also reports of heavy fighting near Ukraine's second city, Kharkiv, in the northeast, where Russian troops blew up a natural gas pipeline, a Ukrainian state agency said. The gas blast there sent a mushroom cloud up into the darkness.

"The enemy wants to destroy everything," said the mayor of Vasylkiv, Natalia Balasinovich.

Russian-backed separatists in the eastern province of Luhansk said a Ukrainian missile had blown an oil terminal in the town of Rovenky.

Russian President Vladimir Putin launched what he called a special military operation on Thursday, ignoring weeks of Western warnings and saying the "neo-Nazis" ruling Ukraine threatened Russia's security - a charge Kyiv and Western governments say is baseless propaganda.

Reuters witnesses in Kyiv reported occasional blasts and gunfire in the city on Saturday night but it was not clear where this was coming from.

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

A US defence official said Ukraine's forces were putting up "very determined resistance" to Russia's air, land and sea advance, which has sent hundreds of thousands of Ukrainians fleeing westwards, clogging major highways and railway lines.

The United States and its European partners said they also would impose restrictions on Russia's central bank to limit its ability to support the rouble and finance Putin's war effort.

"We are resolved to continue imposing costs on Russia that will further isolate Russia from the international financial system and our economies," said a statement from the United States, France, Germany, Canada, Italy, Britain and the European Commission.

After initially shying away from such a move largely because of concern about the impact on their economies, the allies said they committed to "ensuring that selected Russian banks are removed from the SWIFT messaging system."

They did not name the banks that would be expelled, but an EU diplomat said some 70 per cent of the Russian banking market would be affected.

The decision - which the French finance minister had called a "financial nuclear weapon" because of the damage it would inflict on the Russian economy - deals a blow to Russia's trade and makes it harder for its companies to do business.

SWIFT, a secure messaging network that facilitates rapid cross-border payments, said it was preparing to implement the measures.

Sanctions on Russia's central bank could limit Putin's use of his more than $US630 billion ($A871 billion) in international reserves, widely seen as insulating Russia from some economic harm.

Google barred Russia's state-owned media outlet RT and other channels from receiving money for ads on their websites, apps and YouTube videos, similar to move Facebook made.

The Kremlin said its troops were advancing again "in all directions" after Putin ordered a pause on Friday. Ukraine's government said there had been no pause.

Particularly in northern Ukraine, Russia's forces "have been frustrated by what they have seen is a very determined resistance," the US official said, without providing evidence.

A Ukrainian presidential adviser said about 3500 Russian soldiers had been killed or wounded. Western officials have said intelligence showed Russia suffering higher casualties than expected.

Russia has not released casualty figures and it was impossible to verify tolls or the precise picture on the ground.

At least 198 Ukrainians, including three children, have been killed and 1115 people wounded, Interfax quoted Ukraine's health ministry as saying.

Ukraine, a democratic nation of 44 million people, won independence from Moscow in 1991 after the fall of the Soviet Union and wants to join NATO and the EU, goals Russia opposes.

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.

UN refugee chief Filippo Grandi said more than 150,000 Ukrainian refugees had crossed into Poland, Hungary, Moldova and Romania.

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Huge explosions from Russian attacks on oil and gas installations lit up the night sky in Ukraine, while Western allies tightened sanctions to banish major Russian banks from the main global payments system.

Ukrainian forces were holding off Russian troops advancing on the capital Kyiv, President Volodymyr Zelenskiy said as the biggest assault on a European state since World War Two entered a fourth day.

But the night was brutal, with shelling of civilian infrastructure and targets including ambulances, Zelenskiy said on Sunday.

Casualties from the war are unclear. A United Nations agency reported 64 civilian deaths and Ukraine claimed to have killed 3500 Russian soldiers.

More than 100,000 refugees, mainly women and children, have poured into neighbouring countries, clogging railways, roads and borders since Russian President Vladimir Putin launched what he called a special military operation on Thursday.

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.

Offering a glimmer of hope for talks, the Kremlin sent a diplomatic delegation to neighbouring Belarus. Ukraine quickly rejected the offer, saying Belarus had been complicit in the invasion.

However, Zelenskiy left the door open for "real negotiations," elsewhere, an adviser said.

Russian missiles found their mark overnight, including a strike that set an oil terminal ablaze in Vasylkiv, southwest of Kyiv, the town's mayor said. Blasts sent huge flames and billowing black smoke into the night sky, online posts showed.

"The enemy wants to destroy everything," said the mayor, Natalia Balasinovich.

Heavy fighting took place for Ukraine's second city, Kharkiv, in the northeast, where Russian troops blew up a natural gas pipeline, a Ukrainian state agency said.

That blast sent a cloud up into the darkness, though Ukraine's gas pipeline operator said the transit of Russian gas to Europe via Ukraine was going on as normal.

Kremlin-controlled energy giant Gazprom also said that Russian gas exports via Ukraine to Europe continued normally.

Russian troops later entered Kharkiv, interior ministry adviser Anton Herashchenko said on Telegram. Videos posted by him and a state agency showed several military vehicles moving on a street and, separately, a burning tank.

Russian-backed separatists in the eastern province of Luhansk said a Ukrainian missile had blown up an oil terminal in the town of Rovenky.

Reuters witnesses in Kyiv reported occasional blasts and gunfire through the night, then three blasts after air raid sirens went off shortly before 9am (1600 AEST).

Ukrainian leaders were defiant.

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

A US defence official on Saturday said Ukraine's forces were putting up "viable" resistance to Russia's air, land and sea advance.

The United States and its allies have authorised 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.

On Saturday, they 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 said they would 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

They did not name the banks that would be expelled. An EU diplomat said some 70 per cent of the Russian banking market would be affected.

Sanctions on Russia's central bank could limit Putin's use of his more than $US630 billion ($A871 billion) in international reserves, widely seen as insulating Russia from some economic harm.

Google barred Russia's state-owned media outlet RT and other channels from receiving money for ads on their websites, apps and YouTube videos, similar to a move by Facebook.

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 NATO and the EU, goals Russia opposes.

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.

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Western allies have announced sweeping new sanctions against Russia, including banishing its key banks from the main global payments system, as Ukraine's forces attempt to repel Russian troops advancing on Kyiv.

Russian forces pounded several cities with missiles overnight, setting an oil terminal ablaze in the town of Vasylkiv, southwest of the capital, Kyiv, its mayor said.

The blasts sent flames and smoke into the night sky, online posts showed.

"The enemy wants to destroy everything," said the mayor, Natalia Balasinovich.

The Ukrainian president's office also said Russian forces blew up a gas pipeline in Kharkiv, the country's second-largest city.

Residents there were advised to cover their windows with damp cloth or gauze and drink plenty of fluids.

Russian President Vladimir Putin launched what he called a special military operation on Thursday, ignoring weeks of Western warnings and saying the "neo-Nazis" ruling Ukraine threatened Russia's security - a charge Kyiv and Western governments say is baseless propaganda.

Reuters witnesses in Kyiv reported occasional blasts and gunfire in the city late on Saturday but it was not clear where it was coming from.

"We have withstood and are successfully repelling enemy attacks. The fighting goes on," Ukrainian President Volodymyr Zelenskiy said in a video message from the streets of Kyiv posted on his social media.

A US defence official said Ukraine's forces were putting up "very determined resistance" to the three-pronged Russian advance that has sent hundreds of thousands of Ukrainians fleeing westwards, clogging major highways and railway lines.

The US and its European partners also said on Saturday they would impose restrictions on Russia's central bank to limit its ability to support the rouble and finance his war effort.

"We are resolved to continue imposing costs on Russia that will further isolate Russia from the international financial system and our economies," said a joint statement from the United States, France, Germany, Canada, Italy, Britain and the European Commission.

After initially shying away from such a move, the allies said they were committed to "ensuring that selected Russian banks are removed from the SWIFT messaging system".

They did not name the banks that would be expelled, but an EU diplomat said some 70 per cent of the Russian banking market would be affected.

SWIFT said on Saturday it was preparing to implement the new measures in coming days.

"We are engaging with European authorities to understand the details of the entities that will be subject to the new measures and we are preparing to comply upon legal instruction," it said in a statement.

The decision - which the French finance minister had earlier called a "financial nuclear weapon" because of the damage it would inflict on the Russian economy - deals a blow to Russia's trade and makes it harder for its companies to do business.

Sanctions on Russia's central bank could limit Putin's use of his more than $US630 billion ($A871 billion) in international reserves, widely seen as insulating Russia from some economic harm.

The new measures will prevent Russia from "using its war chest", said Ursula von der Leyen, president of the European Commission, the European Union's executive.

But because Russia's large banks are deeply integrated into the global financial system, such sanctions could have a spillover effect, hurting trading partners in Europe and elsewhere.

Ukrainian Prime Minister Denys Shmygal praised the sanctions in a Twitter post on Sunday.

"Thanks to our friends ... for the commitment to remove several Russian banks from SWIFT," he posted.

The Kremlin said its troops were advancing again "in all directions" after Putin ordered a pause on Friday. Ukraine's government said there had been no pause.

A Ukrainian presidential adviser said about 3500 Russian soldiers had been killed or wounded. Western officials have also said intelligence showed Russia suffering higher casualties than expected and its advance was slowing.

Russia has not released casualty figures, and it was impossible to verify tolls or the precise picture on the ground.

The US official said Russia's forces had not made the progress that they wanted to, particularly in the north.

At least 198 Ukrainians, including three children, have been killed and 1115 people wounded so far, Interfax quoted Ukraine's Health Ministry as saying.

With additional reporting by AP

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