A defiant President Volodymyr Zelenskiy says Kyiv remains under Ukrainian control as Russian forces renewed their assault, pounding the capital and other cities with artillery and cruise missiles.
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.
But he is also welcomed efforts to open talks with Russia.
In the video, Zelenskiy said that Turkish President Recep Tayyip Erdogan and Azerbaijani President Ilham Aliyev had offered to help organise talks and that "we can only welcome that".
Diplomatic efforts to end the bloodshed have so far faltered.
Zelenskiy offered on Friday to negotiate a key Russian demand: that Ukraine declare itself neutral and abandon its ambition of joining the NATO military alliance.
But movement to actually advance any diplomacy has appeared to sputter.
Russian President Vladimir Putin launched what he called a special military operation before dawn on Thursday, ignoring warnings from the US and its allies and saying the "neo-Nazis" ruling Ukraine threatened Russia's security.
The Kremlin said its troops were advancing again after Putin ordered a pause on Friday for anticipated talks that never happened.
An adviser to Zelenskiy denied that Ukraine had refused negotiations but said Russia had attached unacceptable conditions.
He also said it was untrue that Russia had paused troop movements.
The crisis has galvanised NATO, which has announced a series of moves to reinforce its eastern flank.
While NATO 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.
The United States has observed more than 250 launches of Russian missiles, mostly short-range, at Ukrainian targets, the US defence official said.
"We know that (Russian forces) have not made the progress that they wanted to make, particularly in the north. They have been frustrated by what they have seen is a very determined resistance," the official said, without providing evidence.
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 that saboteur groups were active.
Klitschko, a former world heavyweight boxing champion, said 35 people including two children had been wounded overnight and that he was imposing a curfew from Saturday evening until Monday morning.
Ukrainians faced lengthy queues for money at cash machines and for fuel at petrol stations, where individual sales are mostly limited to 20 litres.
Many shops in the city centre were closed and the streets were largely empty on Saturday afternoon.
"I was smart enough to stock up food for at least a month," said Serhiy, out for a walk before the curfew.
"I did not trust the politicians that this would end peacefully."
At least 198 Ukrainians, including three children, have been killed and 1115 people wounded so far in Russia's invasion, Interfax quoted Ukraine's Health Ministry as saying.
It was unclear whether the numbers comprised only civilian casualties.
Interfax later cited the regional administration in Donetsk, eastern Ukraine, saying 17 civilians had been killed and 73 wounded by Russian shelling.
Russia says it is taking care not to hit civilian sites.
A Ukrainian presidential adviser said about 3500 Russian soldiers had been killed or wounded.
Russia's Defence Ministry said its forces had captured Melitopol, a city of 150,000.
Ukrainian officials did not comment and the UK cast doubt on the report.
If confirmed, it would be the first significant population centre the Russians have seized.
The city of Mariupol, a key port on the Sea of Azov in southeast Ukraine, remained under relentless shelling on Saturday, its mayor Vadim Boychenko said in a televised address.
"They are shelling schools, apartment blocks," he said.
About 100,000 people have crossed into Poland from Ukraine since Thursday, including 9000 who have entered since 7am on Saturday, Polish Deputy Interior Minister Pawel Szefernaker said.
Ukrainians were also crossing the borders into Hungary, Romania and Slovakia.
The US and European Union have also announced new sanctions on Russia, including blacklisting its banks and banning technology exports.
They have stopped short of forcing Russia out of the SWIFT system for international bank payments, although Germany's foreign and economy ministers and a French presidential official indicated on Saturday that such a move may come soon.
Several European countries, including Russia's Baltic neighbours Lithuania and Latvia, said they were closing their airspace to Russian airliners.
In one of the first visible signs of sanctions being enforced over the invasion, France seized a car cargo ship in the Channel on Saturday that has been linked to the son of a former Russian spy chief.
with reporting from AP
© DPA 2022
Image: Ministry of Defense of Ukraine/Flickr