A Russian air strike on Wednesday severely damaged a paediatric and maternity hospital in the besieged Ukrainian port city of Mariupol, injuring at least 17 people.
Ukrainian President Volodymyr Zelensky said that adults and children were "under the wreckage," in a video he posted on Twitter.
Conditions in the city have been described as "apocalyptic".
Ukraine evacuated more than 40,000 people in one day on Wednesday but struggled to get civilians away from conflict zones around the cities of Kyiv, Kharkiv and Mariupol, one of the negotiators in the Russia talks said.
It comes after a reported pledge from Russia to half its shelling of these cities to allow for the evacuations.
Besieged cities such as Mariupol and Kharkiv have been subject to a hail of Russian bombing, with desperate attempts to evacuate civilians during a ceasefire on humanitarian corridors between 9am and 9pm Wednesday.
There are fears of further bloodshed with Russia targeting civilians using these humanitarian corridors in the past.
Ukrainian Deputy Prime Minister Iryna Vereshchuk said that Russia had agreed to half its shelling of these areas on Wednesday and warned Moscow against breaking the truce.