"You can't imagine how horrible the conditions were." Oleksandr and Olena are two of the lucky few who recently managed to escape from Mariupol, which is now almost under full Russian control after weeks of bombardment. The city is effectively sealed off from the world, and information about what is happening inside is difficult to confirm independently.But the pair, and others, have given chilling accounts of life in Russia's so-called filtration camps, set up outside Mariupol to house civilians before they are evacuated. Oleksandr and Olena, speaking from the relatively safe western city of Lviv, say they ended up at one of the centres when they tried to escape the city. After walking from their home to an evacuation point, they were driven to a Russian refugee hub at a former school in the village of Nikolske, north-west of Mariupol. "It was like a true concentration camp," Oleksandr, 49, says. The centres have been compared by Ukrainian officials to those used during Russia's war in Chechnya, when thousands of Chechens were brutally interrogated and many disappeared. Oleksandr and Olena were fingerprinted, photographed from all sides, and interrogated for several hours by Russian security officers - "like in a prison", he says. They worried that the Russians would look at their phones, and so they cleared all evidence from their devices of anything to do with Ukraine - including photos of their daughter in front of a Ukrainian flag. They were right to worry. Oleksandr says that during their interrogation, Russian security officers examined photographs, phone call history and contact numbers on their devices for links with journalists or government and military officials. "If a person was suspected of being a 'Ukrainian Nazi', they took them to Donetsk for further investigation or murder," says Oleksandr, although the BBC has not been able to verify this claim. "It was very dangerous and risky. Any small doubt, any small resistance - and they could take you to the basements for interrogation and torture. Everybody was afraid to be taken to Donetsk." President Vladimir Putin has stated one of the aims of his invasion is to clear Ukraine of Nazis, and Russian propaganda has made numerous baseless allegations that Ukraine is somehow aligned with Nazism.Eventually the pair were detained and put on a list for evacuation. But the ordeal did not stop there.