"We assume that they could be there, because the missing himself or unknown people called relatives from Russian numbers, from Chechnya or Adygea. Some of the relatives received calls from human rights activists from Russia with a message that they had found their relatives in a Russian jail. Some of them, as we know, were sent to Chechnya or Adygea," the letter says.

Not only the Ukrainian military but also civilians are among the missing, namely volunteers who provided humanitarian assistance to the needy, and those who visited their relatives who were called for service in the Ukrainian army during the mobilization.

Those who managed to escape from captivity told relatives of the missing people that since the summer the prisoners had been repeatedly transported to Russia’s Rostov region or deeper into Russia to work. They were transported across the border through different checkpoints, in particular through the Izvaryne checkpoint.

"Please note that we are not talking about prisoners of "republics" and "Cossacks." These missing persons are not on the list of dead or among the prisoners to be exchanged in accordance with the Minsk agreements. At the moment, we’ve lost contact with them," the letter reads.

Some relatives of the missing persons were offered by unknown persons by telephone or via the volunteers to return a prisoner for a large ransom. Some callers talked with an Eastern accent.

"Sometimes it sounded like this: "Your relatives were sold into slavery, and you will never see them," the relatives of missing persons wrote.

"All those who were taken to the Caucasus - Chechnya, Ossetia and Adygea - are not likely to be held in a jail. From various sources we know that our relatives were taken and sold as a labor force. We cannot say with certainty that this is true, but such cases have been revealed by the various human rights activists and journalists," the letter says.

At the request of relatives, the journalist Anna Nemtsova tried to trace such a trail in Chechnya. Her sources have confirmed that these events are taking place, but it is necessary to look for these “sold” people - and it's not easy.

Relatives of prisoners have compiled a list of missing persons whose relatives or friends received calls from phones on roaming in Russia, particularly in Chechnya, Ossetia and Adygea. The list consists of 120 people, but apart from them, from 400 to 700 citizens of Ukraine may be in Russia, according to various estimates.