Ukrainian forces are putting up "very determined resistance" to Russia's invasion, according to US defence officials, with President Volodymyr Zelenskiy striking a defiant tone on the streets of the capital Kyiv.
Russian forces have continued to pound Kyiv and other cities with artillery and cruise missiles in a campaign that has sent hundreds of thousands of Ukrainians fleeing westwards towards the European Union, clogging major highways and railway lines.
Top Russian security official and ex-president Dmitry Medvedev said military operations would be waged relentlessly until President Vladimir Putin's goals were achieved, ratcheting up Moscow's rhetoric.
Putin launched what he called a special military operation on Thursday, ignoring Western warnings and saying the "neo-Nazis" ruling Ukraine threatened Russia's security. The assault threatens to upend Europe's post-Cold War order.
Medvedev said new sanctions on Russia were a sign of the West's impotence in the conflict and he hinted at a severing of diplomatic ties, saying it was time to "padlock the embassies".
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.
The Kremlin said Putin had ordered troops to stop advancing on Friday but they were moving forwards again on Saturday after Kyiv refused to negotiate.
An adviser to Zelenskiy denied that Kyiv had refused negotiations but said Russia had attached unacceptable conditions. He also said it was untrue that Russia had paused troop movements on Friday.
Turkish President Tayyip Erdogan, who has fostered good ties with Russia and Ukraine, told Zelenskiy by phone on Saturday that Ankara was making efforts for an immediate ceasefire.
'We have the courage'
Kyiv's mayor Vitali Klitschko said there was currently no major Russian military presence in the capital, but that saboteur groups were active. The metro system serves now only as a shelter for citizens and trains have stopped running, he said.
Klitschko said 35 people, including two children, had been wounded overnight and that he was extending an overnight curfew that kicked in at 5 pm local time.
"We have withstood and are successfully repelling enemy attacks. The fighting goes on," Zelenskiy said in a video message posted on his social media. "We have the courage to defend our homeland, to defend Europe."
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 a 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.
Ukraine, a democratic nation of 44 million people, won independence from Moscow in 1991 and wants to join NATO and the EU, goals Russia opposes. Putin says Ukraine is an illegitimate state carved out of Russia, a view Ukrainians see as aimed at erasing their distinctive history and identity.
Ukraine said more than 1000 Russian soldiers had been killed. Russia did not release casualty figures.
Russia's Defence Ministry said its forces had captured Melitopol, a city of 150,000 in southeastern Ukraine. Ukrainian officials were not immediately available to comment and Britain 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.
Moscow says it is taking care not to hit civilian sites.
Putin has said he must eliminate what he calls a serious threat to his country from its smaller neighbour and has cited the need to "denazify" Ukraine's leadership, accusing it of genocide against Russian-speakers in eastern Ukraine - a charge dismissed by Kyiv and its Western allies as baseless propaganda.
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.