KYIV - A defiant President Volodymyr Zelenskiy said Ukrainian forces were repelling Russian troops advancing on Kyiv on Saturday, as Western nations said they were cutting off a number of Russian banks from the world's main financial payments system, ratcheting up economic pressure on Moscow.

Reuters witnesses in Kyiv reported occasional blasts and gunfire in the city on Saturday evening, but it was not clear exactly where it was coming from. The capital and other cities have been pounded by Russian artillery and cruise missiles.

As the fighting continued, the United States and its allies said they were imposing a new round of sanctions in response to Russian President Vladimir Putin's invasion of Ukraine, including expelling some Russian banks from the SWIFT payment system.

The move deals a blow to Russian trade and makes it harder for Russian 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.

The sanctions, agreed with the United States, France, Canada, Italy, Great Britain and the European Commission, also include limiting the ability of Russia's central bank to support the rouble.

It marked an escalation of the West's punitive economic response. Putin launched what he called a special military operation before dawn on Thursday, ignoring Western warnings and saying the "neo-Nazis" ruling Ukraine threatened Russia's security.

A U.S. defense 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.

• Ukraine has 'derailed' Russian attack plan: Zelensky

"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.

The Kremlin said its troops were advancing again "in all directions" after Putin ordered a pause on Friday.

Russia's assault is the biggest on a European state since World War Two and threatens to upend the continent's post-Cold War order.

• NATO's 'historic' deployment to defend E. Europe