Russian forces appear to have made progress from north-east Ukraine in their slow fight to reach the capital, Kyiv, while one city already under siege was shelled so heavily residents could not bury the growing number of dead.
In past offensives in Syria and Chechnya, Russia's strategy was to crush armed resistance with sustained air strikes and shelling that levelled population centres.
That kind of assault has cut off Ukraine's southern port city of Mariupol, and a similar fate could await Kyiv and other parts of the country if the war continues.
In Mariupol, unceasing barrages have thwarted repeated attempts to bring in food and water and to evacuate trapped civilians.
Mariupol's death toll has passed 1,500 in 12 days of attacks, the Mayor's office said.
A strike on a maternity hospital in the city of 446,000 this week that killed three people sparked international outrage and war-crime allegations.
Continued shelling forced crews to stop digging trenches for mass graves, so the "dead aren't even being buried," the Mayor said.
Mariupol mosque shelled while 80 hid inside
Russian forces have shelled a Mariupol mosque where more than 80 adults and children, including Turkish citizens, have taken refuge, according to Ukraine's foreign ministry.
"The mosque of Sultan Suleiman the Magnificent and his wife Roxolana (Hurrem Sultan) in Mariupol was shelled by Russian invaders," the foreign ministry said in a tweet.
"More than 80 adults and children are hiding there from the shelling, including citizens of Turkey."
It did not say if there were any people killed or wounded.
Moscow has denied targeting civilian areas in what it calls a special military operation in Ukraine.
'More than a dozen hospitals attacked'
Russian forces have hit more than a dozen hospitals since they invaded Ukraine on February 24, according to the World Health Organization (WHO).
Ukrainian officials reported on Saturday that heavy artillery damaged a cancer hospital and several residential buildings in Mykolaiv, a city 489 kilometres west of Mariupol.
The hospital's head doctor, Maksim Beznosenko, said several hundred patients were in the facility during the attack but no one was killed.
The invading Russian forces have struggled far more than expected against determined Ukrainian fighters.
But Russia's stronger military threatens to grind down the defending forces, despite an ongoing flow of weapons and other assistance from the West for Ukraine's westward-looking, democratically elected government.
The conflict has already sent 2.5 million people fleeing the country.
Thousands of soldiers on both sides are believed to have been killed along with many Ukrainian civilians.
On the ground, the Kremlin's forces appeared to be trying to regroup and regain momentum after encountering tough resistance and amassing heavy losses over the past two weeks.
Five bodies found in Kharkiv apartment block
Russian forces were blockading Kharkiv, Ukraine's second-largest city, even as efforts have been made to create new humanitarian corridors around it and other urban centres so aid can get in and residents can get out.
Ukraine's emergency services reported on Saturday that the bodies of five people — two women, a man and two children — were pulled from an apartment building that was struck by shelling in Kharkiv.
The Russians also stepped up attacks on Mykolaiv, 470 kilometres south of Kyiv, in an attempt to encircle the city.
As part of a multi-front attack on the capital, the Russians' push from the north-east appeared to be advancing, a US defence official said.
Combat units were moved up from the rear as the forces advanced to within 30 kilometres of Kyiv.
New commercial satellite images appeared to capture artillery firing on residential areas that stood between the Russians and the capital.
The images from Maxar Technologies showed muzzle flashes and smoke from big guns, as well as impact craters and burning homes in the town of Moschun, 33 kilometres from Kyiv, the company said.
'Positive developments' in Russia-Ukraine talks
With the invasion into its 17th day, Russian President Vladimir Putin said there had been "certain positive developments" in ongoing talks between Russian and Ukrainian negotiators, without giving details.
Ukrainian President Volodymyr Zelenskyy appeared on video to encourage his people to keep fighting.
"It's impossible to say how many days we will still need to free our land, but it is possible to say that we will do it," he said from Kyiv.
Mr Zelenskyy said authorities were working on establishing 12 humanitarian corridors and trying to ensure food, medicine and other urgently needed basics get to people across the country.
He said only 7,000 people were evacuated from four Ukrainian cities on Friday, a sharply lower number than managed to leave in each of the two previous days.
He also accused Russia of kidnapping the mayor of one city, Melitopol, calling the abduction "a new stage of terror".
The US said Russia has launched nearly 810 missiles into Ukraine.
Until recently, Russia's troops had made their biggest advances on cities in the east and south while struggling in the north and around Kyiv.
They also have started targeting areas in western Ukraine, where large numbers of refugees have fled.
Wires/ABC
