Lawyers and members of the Public Monitoring Commission in Angara, who visited Sentsov in Detention Center #1 in Irkutsk, said on September 11 that he was in solitary confinement in the jail's basement.
Lawyer Svyatoslav Khromenkov told RFE/RL that the Irkutsk detention center was considered one of the worst jails in Siberia, but he added that Sentsov's cell was recently repaired and was "more or less OK."
Khromenkov said Sentsov did not know why he was moved so far.
Read alsoPoroshenko: Ukraine with whole world to fight for Sentsov's releaseKhromenkov suggested Sentsov may have been transferred to prevent him from staying at one detention facility long enough to establish himself there.
Inmates in the former Soviet Union are typically moved from detention facility to another in so-called Stolypin train carriages that are specially equipped for the transportation of convicts.
Transfers from one jail to another, especially long-distance transfers, typically have been used to put pressure on some inmates.
Convicts are forced to spend days in narrow cages inside the train cars, which very often are jammed with people. There often is no fresh air nor the possibility to use a toilet on the train.
Long distances are covered in many days because Stolypin trains are only allowed to move during certain hours, usually at night.
Read alsoUkraine MFA verifying reports on Sentsov's transfer to Russian high-security prisonThat means prisoners are kept either inside the railroad cars at stations or in transit jails, making such trips long and exhausting.
Sentsov, a native of Crimea who opposed Russia's seizure and annexation of the Ukrainian peninsula, is currently serving a 20-year prison term on terrorism charges.
He and international human rights groups say the charges are politically motivated.
Sentsov was arrested in May 2014 on suspicion of planning the fire bombings of pro-Russian organizations in Crimea. A Russian court convicted him on multiple terrorism charges in August 2014.
Sentsov has denied all charges against him, saying that a "trial by occupiers cannot be fair by definition."