KYIV, Ukraine – Nineteen children, most of them orphans, have lived in harrowing conditions after being stuck in a clinic in Ukraine's besieged city of Mariupol for weeks, a carer, a witness and a charity told AFP.

The witness who managed to get out, Alexei Voloshchuk, told AFP that the children were in "huge danger".

The children, aged four-17, were sent to a sanatorium for minors that specialises in pulmonary conditions from their homes in nearby towns before Russian President Vladimir Putin launched his invasion of Ukraine on February 24th.

Their carers have been unable to get to them since due to heavy fighting in the port city that has been severely destroyed by Russian forces.

Mariupol has seen a near total communications blackout and it is not possible to speak to the children. The city has also been cut from water, gas and electricity.

Witness Voloshchuk had taken shelter in the sanatorium, where his relative worked.

Speaking from the central city of Zaporizhzhia where he was evacuated to on Friday, he said the children were living in freezing cellars and had not washed for more than two weeks.

Voloshchuk sent AFP a picture of the children that he had taken, which showed windows of the clinic shattered. He said rockets had landed nearby.

"There is no heating, it is cold. One of the girls, around eight-years-old, showed me a wound on her face, she said it was from the cold," he said.

The children were being looked after by a "hero" pulmonologist, a cook and two nurses, he said.

Voloshchuk -- who moved back to his native Mariupol from Kyiv in December, weeks before Putin's invasion -- said local policemen had taken the children "under their wing" and were bringing them food, which is being cooked on fires outside.

But he worried that supplies may run out soon.

- 'Desperate' to get them out -

One of the children's carers, Olga Lopatkina, said she sent six of her foster children to the sanatorium in January.

"They are aged six, eight, 11, two 12 year-olds and 17," she said.

She runs a private foster care home with her husband in the town of Ugledar, 100 kilometres (60 miles) north of Mariupol.

When fighting erupted, Lopatkina took her remaining children west, moving to the city of Lviv first, before going to Hungary from where volunteers helped her get to France.

She desperately wants her foster children to join her.

Mariupol residents can currently only be evacuated by private cars.

A health charity, Geneva-based Stop TB Foundation, said it was "horribly concerned" about the children.

Its executive director Lucica Ditiu told AFP Friday that she was "personally desperate" to get the children out.

She said the charity had tried to ensure that the children would be taken by other countries, "but the biggest problem is to take them out of there".

"We always speak about 'let's not leave anyone behind'," she said, adding that "they are orphans... the most vulnerable".

She also praised those who stayed behind to care for the children.

"I cannot even imagine what the situation is to have 19 kids with you in a basement with disappearing food and water."