A maternity hospital in the southern port city of Mariupol has been hit by a Russian air strike, Ukraine says.
President Volodymyr Zelensky said people were trapped under the wreckage, and called on Western leaders to impose a no-fly zone.
He also posted footage apparently from inside the hospital, which appears badly damaged.
A regional official told Ukrainian media that at least 17 people were injured, including staff and patients.
Pavlo Kyrylenko, the head of the Donetsk regional administration which includes the city of Mariupol, said no deaths had been confirmed, and there were no confirmed injuries amongst children. He said the attack happened during an agreed ceasefire with the Russian side, according to Interfax Ukraine.
The Mariupol city council said the strike had caused "colossal damage", and published footage showing burned out buildings, destroyed cars and a huge crater outside the hospital. The BBC has verified the location of the videos.
In a tweet, UK Prime Minister Boris Johnson said that "there are few things more depraved than targeting the vulnerable and defenceless".
Mariupol has been surrounded by Russian forces for several days, and repeated attempts at a ceasefire to allow civilians to leave have broken down.
"The whole city remains without electricity, water, food, whatever and people are dying because of dehydration," Olena Stokoz of Ukraine's Red Cross told the BBC, adding that her organisation would continue trying to organise an evacuation corridor.
Earlier Ukrainian Foreign Minister Dmytro Kuleba said that Russia continued to hold more than 400,000 people "hostage".
Russia insists that it does not target civilian infrastructure in Ukraine.
Meanwhile, convoys of civilians are due to leave several towns near the Ukrainian capital, Kyiv, according to regional leader Oleksiy Kuleba.
Ukraine's armed forces agreed to stop firing on Wednesday along six evacuation routes for 12 hours, from 09:00 to 21:00 local time (07:00 to 19:00 GMT).
They urged Russian forces to fulfil their commitment to the local ceasefires, but Russian shelling has continued with further reports of civilian deaths.
Oleksiy Kuleba, who heads Kyiv's regional administration, said civilians were due to leave towns such as Bucha and Hostomel to the north-west of Kyiv, several of them under Russian occupation. "I really hope everything will be fine and we'll look after our people."
Police said convoys of buses and other vehicles were on their way and they asked residents to follow their instructions. In a separate incident they said a policeman had been shot dead by Russian forces as he helped evacuate civilians from the town of Demydiv north of Kyiv.
The mayor of Russian-controlled Enerhodar in the south said civilians had begun boarding buses to leave, but in the north-east officials said another key corridor from Izyum, southeast of Kharkiv, had to be halted because of Russian bombardment.
A woman called Valentina with relatives in the northern city of Chernihiv told the BBC that Ukrainian soldiers had prevented a column of civilians leaving by car on Tuesday.
She spoke to her relatives early on Wednesday: "They were turned back by Ukrainians and were told there were battles on the road. Today they tried again. Yesterday somebody came in the other direction along the road, so it wasn't closed." She also said the people in Chernihiv, hiding in cellars from daily Russian missile attacks, had not been told about the evacuation corridors.
Ukraine managed to carry out its first mass evacuation on Tuesday, bussing an estimated 5,000 civilians from Sumy to Poltava, in the centre. The convoy took almost 12 hours to reach the city, forced to drive a long way round areas of active fighting.
BBC correspondent Sarah Rainsford watched the arrivals leave crowded buses and rush for a waiting train. The convoy took almost 12 hours to reach the city, forced to drive a long way round areas of active fighting.
Many on the first convoy out of Sumy were medical students from India. One woman described two "miserable" weeks living mostly underground with dwindling supplies of water and food. "We were starving," Manisa said.