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WATCH: The aftermath of a Russian strike on a maternity and children's hospital in Mariupol.

A maternity and children's hospital in the 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 appeared 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 port 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.

"We don't understand how it's possible in modern life to bomb a children's hospital. People cannot believe that it's true," Mariupol Deputy Mayor Serhiy Orlov told the BBC.

The White House condemned the "barbaric" use of force against innocent civilians, and UK Prime Minister Boris Johnson tweeted 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.

Deputy Mayor Orlov said at least 1,170 civilians had been killed in the city since Russia began its bombardment, and that 47 people there were buried in a mass grave on Wednesday, although those figures have not been independently verified.

The UN says it has verified 516 civilian deaths across Ukraine, but it believes the real figures are "considerably higher".

Russia insists that it does not target civilian infrastructure in Ukraine.

Humanitarian convoys

Meanwhile, convoys of civilians were 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 continued with further reports of civilian deaths.

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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.

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.

Image source, Ukrainian police

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A convoy of buses escorted by the police set out on Wednesday to take part in the evacuations to Kyiv

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.

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WATCH: Civilians flee Sumy, which is close to the Russian border and frontline