KYIV -- Western allies have announced sweeping new sanctions against Russia, including banishing its key banks from the main global payments system, drawing thanks from Ukraine on Sunday as its forces were repelling 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.

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.

The biggest assault on a European state since the Second World War threatens to upend the continent's post-Cold War order.

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 Zelensky said in a video message from the streets of Kyiv posted on his social media.

A U.S. 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 United States and its European partners also said 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," the Western allies said as they escalated their punitive response.

"We will implement these measures within the coming days," according to a joint statement from the United States, France, Germany, Canada, Italy, Britain and the European Commission. 

'THANKS TO FRIENDS'

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% of the Russian banking market would be affected.

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.

SWIFT, or the "Society for Worldwide Interbank Financial Telecommunication," is a secure messaging network that facilitates rapid cross-border payments, making it a crucial mechanism for international trade.

Sanctions on Russia's central bank could limit Putin's use of his more than US$630 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 Shmyhal said in a Twitter post on Sunday: "Thanks to our friends ... for the commitment to remove several Russian banks from SWIFT."

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 3,500 Russian soldiers had been killed or wounded. Western officials have also said intelligence showed Russia suffering higher casualties than expected and its advance slowing.

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

'DETERMINED RESISTANCE'

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

"They have been frustrated by what they have seen is a very determined resistance," the U.S. official said, without providing evidence.

At least 198 Ukrainians, including three children, have been killed and 1,115 people wounded so far, Interfax quoted Ukraine's Health Ministry as saying.

Interfax later cited the regional administration in Donetsk, eastern Ukraine, saying 17 civilians had been killed and 73 wounded by Russian shelling.

Moscow says it is taking care not to hit civilian sites.

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 have crossed into neighbouring countries -- half to Poland, others to Hungary, Moldova and Romania.

U.S. President Joe Biden approved the release of up to $350 million worth of weapons from U.S. stocks, while Germany, in a shift from its long-standing policy of not exporting weapons to war zones, said it would send anti-tank weapons and surface-to-air missiles. 

(Reporting by Maria Tsvetkova, Aleksandar Vasovic and Natalia Zinets in Kyiv; Alan Charlish in Medyka, Poland; Fedja Grulovic in Sighetu Marmatiei, Romania; and Reuters bureaus; Writing by Robert Birsel, Gareth Jones and Alex Richardson; Editing by William Mallard, David Clarke, Alison Williams and Daniel Wallis)