A defiant President Volodymyr Zelenskiy says Ukrainian forces are repelling Russian troops advancing on Kyiv, as Western nations announce plans to cut off some Russian banks from the world's main financial payments system.
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 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.
"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 II and threatens to upend the continent's post-Cold War order.
While the NATO Western military alliance has said it will not deploy troops to Ukraine, a string of countries are sending military aid.
US President Joe Biden approved the release of up to $US350 million ($A484 million) worth of weapons from US 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.
Ukrainian Vice Prime Minister Mykhailo Fedorov said his government will create an "IT army" to fight against Russia's digital intrusions.
A Ukrainian presidential adviser said about 3500 Russian soldiers had been killed or wounded and Russian troops had not made serious gains on the third day of fighting.
Russia has not released casualty figures and it was impossible to verify tolls or the precise picture on the ground.
Western officials have also said intelligence showed Russia suffering higher casualties than expected and its advance slowing.
Chechen leader Ramzan Kadyrov, a close Putin ally, said on Saturday his fighters were also deployed in Ukraine.
He said Russian forces could easily take Kyiv and other large cities but their task was to avoid loss of life.
Kyiv's mayor Vitali Klitschko said there was no major Russian military presence in the capital, but saboteur groups were active and he was imposing a curfew from Saturday evening until Monday morning.
© DPA 2022